My take:
Davening properly 3 times a day takes A LOT of effort. Not just making it out to minyan, that's the easy part. Truly connecting can be emotionally draining.
Therefore, my observation/assumption is that some people daven properly some of the time.
But when someone stops putting in the emotional effort, the davening feels fake--just going through motions. It's a horrible feeling, and leads many to skip it altogether, or as often as their conscience allows.
As an aside, it seems this is less of problem with the older generations. The 60+ crowd is much better with never missing minyan, even in very modern communities. Perhaps the younger generation is more honest with itself.