Q:

Why is "The Loop" survey asking about race, age, and gender?

While I understand that "The Loop" is not a research instrument in the same line as what a social scientist would develop, it is still a survey instrument and reflects upon us all (i.e., poorly designed surveys actually lead to fewer people taking them overall). As such, most surveys follow the American Association for Public Opinion Research (AAPOR) Best Practices for Survey Research. 
Looking at the survey, I really don't understand why demographic information (i.e., race, age, and gender) are being asked since it appears that the clear goal is to get an initial impression of "how satisfied are you with your experience using Stack Overflow?" While I understand that a lot of people will include these by default, unless it is relevant, it's generally taught in survey methods courses to avoid asking the questions. 
In this case it appears that your sample frame is effectively all developers in which case demographics really aren't needed​ for this particular question. In scientific research, demographics are only requested if there is evidence that a demographic attribute will lead to a significant difference in responses (ex., "Does a respondent's age influence the amount of money they save for retirement?"). Otherwise, demographic questions are left off to avoid respondent fatigue and because paper and ink is expensive!1 
Another reason why this is problematic is that the question is asking about "racial background" but the survey appears to be directed at an international community. Needless to say that from that standpoint, the demographic information looses all of its value as a data point since the respondents don't belong to the same cultural background, i.e., someone from Japan has a vastly different perspective from someone from China even though they may both respond "East Asian." This actually reflects very poorly upon Stack Overflow since it represents an extremely narrow perspective and​ understanding of culture.
If the "The Loop" survey is intended to be a preliminary instrument, then a better way of approaching things would have been to code the qualitative data and develop a new survey instrument (or panel interview more likely) from that. 
As such, per the title, why is "The Loop" survey asking about race, age, and gender?

Obviously that last point isn't relevant online, but you may or may not be surprised at how much work is done to limit the length of a survey to save on printing costs.

A:

Because that is what Corporate actually cares about. Or at least race & gender. Probably age to a lesser degree. The "gender" part has been emphasized recently via the "pronoun problem", but both race & gender have entered into the general world out in so many flavors. Call it "affirmative action". Call it "inclusive". Call it whatever you want.
My personal take on all of this: Programming (and nearly all the SE network of Q&A, but especially the programming and other technical sites)  should be a race, age & gender-agnostic field. For better or worse, that is not always the case, but that should be the goal, not to make it "better" for a historically disaffected group but simply "great" for "everyone".
The question becomes whether to somehow proactively figure out how to "solve" that problem by asking who your users are (which appears to be the case with this survey) and somehow use that information to make the site better (if group 'a' seems happier than group 'b' then figure out how to make things better for 'b' - a noble goal if you can actually do anything about it) or to simply produce a product that has no inherent bias for/against any race, gender or age, where everyone is welcome and nobody cares if you are male, female, non-binary, young, old, white, black, etc. 
SE historically has been a site where you could ask a programming question and (unless you wanted to let people know by way of your username or an actual photo in your profile) nobody would know or care whether you were male, female, non-binary, young, old, white, black, etc. Because C, PHP, Python, Java, etc. do not give different error messages for women and HTML/CSS produces different output in different browsers but not dependent on the age of the user. 
Now if you're trying to sell a company to some big "words are all that matter, not deeds" corporation, then maybe this all matters. If you are trying to simply provide a place where people can ask technical questions and get answers, it doesn't matter AT ALL. Maybe a different system like Codidact (I am one of many people involved in developing Codidact, and there are many other open source Q&A systems out there) will do a better job of that, focusing on the users and not on political correctness.
But I digress.

A:

Race, age and gender are just a part of the larger identity politics that has been seeping into tech.
For a larger context, I refer you to the social psychologist Jonathan Haidt:

Q: Are these problems graduating with Gen Z to the workplace?
H: While we were writing the book, I was beginning to hear anecdotes from business people, but we didn't have enough to go on. By January 2019, it was clear in industries that hire from elite liberal arts colleges. When we talk to people, they'll say young employees are so fragile and come in looking for confrontation. Members of Gen Z, they are used to an environment in which it's all about displaying what side you're on, supporting this person, opposing that. Everything becomes a moral conflict. But you can't run a company that way.
Q: How do we stop this from happening in an office?
H: If corporate culture becomes political, people feel fearful or reluctant to stand up. We need a speak-up culture, but Gen Z has been raised with a call-out culture in which they're incentivized to criticize or shame people publicly. A callout culture makes people feel they're walking on eggshells, and you can't have that in a company.

So there you have it. This is a cultural problem in tech that needs addressing by multiple organizations. Leaders of the companies should start speaking up against the growing divisiveness in tech, and aim to restore meritocracy.

Haidt has summarized a sociological paper to provide a more general sociological perspective into what is happening in tech: Where microaggressions really come from: A sociological account.

A:

it is still a survey instrument

No, it is a diversion, a supposed alternative to actual dialog between the company and the SE community, here on MSE. I suggest Galastel's excellent summary of it.

and reflects upon us all

Cooked surveys which are put forward as a diversion and managed by untrustworthy people/organizations should not be taken seriously.
But I suggest we all make sure and actively undermine this initiative, by boycotting these surveys.

