Convenience, mainly. If I'm given a choice between purchasing and using system compatible adventures vs. non-system compatible adventures, I'm much more likely to go with the system compatible adventures. It's just easier.
(As a side point, there's more to adventure compatibility than just the crunch. Sometimes the design approach of the adventures is different for various systems.)
My original post said I'm more likely to accept new editions that retain broad system compatibility with what has gone before (i.e., they're the "same game" in that sense). The "adventure test" is a way of evaluating that.
This is a good way of thinking about things, in my opinion. I was one of the many people who accidentially ran B2 in AD&D at one point. You eventually find issues but you could forge ahead without ever cracking open a monster manual. Trying to run B2 for 3.5E D&D would be much more challenging and would require some serious rebuilding on the fly.
No monster having a 10 AC isn't that big of a deal; no creature has a will save is much more problematic.
I also want to jump on the bandwagon that not supporting out of print copies of the game isn't helping matters. Other companies do so -- I just noticed that most editions of Stormbringer are available in PDF (despite it being a vey minor game).