It's largely because the folks who kind enough to produce these are subject to real life constraints. More often than not, the progress that's made is slow, particularly with .ini editing, which I believe is where they're at with this. And creating a new FM for a new model can be a complete bastard sometimes given the number of mathematical values involved, not to mention the other variables that have to be entered, tested, tweaked, tested again, and so on, and there's usually not a whole lot to report. Editing ini files are easy enough since the data is already there and it works, but when producing new ones, the testing process has to be thorough otherwise odd things such as this can happen:
An accidental double entry that resulted in my M61A1 shooting me back at Mach 7
Missing the entry for special effects resulting in no afterburner
I can't even guess what's happening here.
My point is that these are things I missed when using them straight away after editing (even after going through the edits and being convinced it'd work) that would basically kill the quality of the plane had I released them (had they been mine to release in the first place).
Don't get me wrong, I hate waiting to the Gripen too, but it'll be worth the wait, especially sans any potential bugs.