The new generation has established its presence and with it the community at large has changed. It takes a little bit of time before the old gen has had enough of being out of touch, but their disdain for changes can blind them to the good stuff. When a community grows, content increases and usually less effort is spent on each individual post/thread. This means, if you preferred how it was before, that you have to filter more content. Once that amount has surpassed what you find interesting, the community starts feeling like it's not a place for you anymore.
Learning to filter more is a matter of habit. It usually takes years to be able to ignore your personal disdain and focus on the harder-to-find gems in the dirt and, before you do, a community you've grown out of touch with takes too much energy to process. It happens easily when you've had to focus on your real life for an extended period of time, and the work load almost always increases after the new year, no matter your profession/school program. It's a process that repeats itself every year, and it's how the community develops.