Apparently it was a point of debate.
I believe the reason was that many users were confused as to how to manage multiple files (and share variables between them), leading to frustration and bad coding practices (such as having static motors duplicated in each file).
Also, it makes more sense to me from a logistical point of view, as
main.cpp becomes the master competition control file which manages all the competition logic. You can still have your functions separated into other files, but it makes sense to call them from a central location.
Last year, I combined the files together as it made more sense to me.
However, there is no reason you can’t keep your files separate (just delete main.cpp) or have multiple files in general. This just changes the default project template to make the transition easier for new programmers coming from VEXcode or RMS, which use a single-file approach.