Why wouldn't a professor allow a student to record audio of his lecture? I am a master’s student in a public university in the state of New York. I do not consider myself to be disabled in any form but I do find it difficult to concentrate in class. I lose focus too often and have difficulty recalling concepts taught during class. So I find resources such as recorded lectures to be very quite beneficial as whenever I lose focus while watching them, I can simply rewind back. 

I am currently enrolled in a course where lectures are not recorded. I like this subject and I want to do well. So I want to ask the Professor permission to audio-record his lecture (by just keeping my phone on my desk and not using any distracting piece of equipment). I don't know the professor well but he definitely isn't one of those overtly friendly ones who (you'd think) will surely give you permission.

What I want to know is if he were to deny me, what could be his reasons?

I think I have just one shot at this. So I want to go there prepared.

A vaguely similar (but not the same) question has been asked before.



Update: I asked the Professor if I could record and he simply smiled and said yes.
This is a subjective question, but common reasons I have heard (and reasons I would have) include:


Fear of students editing records to manipulate what you said
Fear of having a mistake you made taken out of context and spread widely 
Concern that it encourages students to not pay as close of attention 
Concern that students will not feel they need to attend class if they have acceess to the lecture outside of class.
Concern that students in the class will not feel as comfortable asking questions or contributing to discussion, since they are also being recorded
Institutional concern about a course's lectures being widely distributed online, when in fact these lectures are for (paying) enrolled students--a type of "intellectual property" concern 


Edit: Also, you might want to be aware when bringing up problems getting easily distracted in the class, a professor could take that somewhat personally. Many committed teachers go out of their way to try to make their class as interesting and engaging as possible. If a student asked to record lectures because they are getting distracted or zoning out during class, one may not hear: "I am trying to do this to improve my learning;" but rather: "I am trying to compensate for your short-comings as a teacher". Not all instructors will feel this way, but that's just something to keep in mind as you broach that conversation. 

Edit2: There's a lot of discussion in comments that my last comment about socially navigating the question: it is very true that many teachers may not care. However, trying to frame the request more explicitly as "this is a thing I am doing to improve my learning" could be beneficial in making your instructor agree.