Why is there less funding or scholarship opportunity for business students compared to other background students? If one compares the funding state between the business background students and the science, technology, engineering, math (STEM) background students in university, one will surely see much difference in the portion or extent of funding. Scholarship opportunities for business students are rare and small.  Why is this the case?
It may be useful to take a slightly larger perspective, rather than comparing just business vs. STEM.  While there is a great degree of variation around the world and from program to program, most higher-level degree funding can be clustered into three general categories:


Doctoral degrees in STEM fields are generally funded by universities or external agencies.
Terminal "professional" degrees, including MBA, MD, JD, DDS, etc. frequently expect students to be responsible for their own funding, often going deeply into debt to do so.
Doctoral degrees in humanities and liberal arts fields are in-between, where sometimes students are supported or partially supported and other times they are not.


There's a great deal of variation, of course.  For example, I believe that the education of medical doctors is supported by some nations (though I lack a reference at the moment).  Likewise, Masters degrees are sometimes considered a "professional degree" that students are expected to fund, while other times they are lumped in with STEM doctoral degrees and funded by the university or other sources.

So, why should this be the case?  It seems to me that a lot of this is simply independently developing markets:


STEM students are typically expected to contribute significantly to the investigation of science and engineering as part of their training, and since these contributions can be very valuable in those markets, it makes sense for external funding agencies to include students in their funding.
Professional degrees are often coupled with very high post-graduation salaries, while the training period is much more one-way, with the students spending most of their time learning rather than making novel (and readily fundable) contributions.
Humanities and liberal arts are also quite important societally, but they are not currently associated with markets flush with capital and an easy set of incentives to pour money into schools either during the program (as with STEM) or afterward (as with professional degrees).  As such, they have generally developed much more ad hoc and patchwork approaches to funding degrees.