Reach did not bring Halo downhill,there is nothing to understand about that as you are completely wrong. Just because it wasn't adopted by MLG and 343 has repeatedly catered to that crowd with 4 and 5 and the franchise only fell further.
In what universe were Halo 4 and Halo 5 titles that in any way "catered" to the Halo competitive community in any meaningful way? Toning down bloom and removing armor lock? Bringing back descope in Halo 5? Small change compared to the way either of those games doubled down on many of Reach's controversial aspects and went even farther.
Reach may not have had the glaring issues regarding content or lack thereof that Halo 4 and Halo 5 had, but it would be ridiculous to not acknowledge that the deep rifts in the Halo community started with Reach. The grassroots Halo community dying with Reach is just one symptom. Regardless of how anyone feels about any one change like armor abilities, trying to deny that Reach was the start of the downward trend is just absurd.
The series peaked in popularity with Halo 3 and has been steadily going downhill ever since. While I'm not one to say correlation equals causation, I do tend to think it is worth looking into.