To me, writing in full, formally correct sentences, being careful to always use correct punctuation, starts to feel a little pretentious or tryhard in some contexts.

It doesn't feel too much like that here on HN. But on reddit, I use less formal structure most of the time, and that feels natural to me.

I usually speed read and the shape of the words helps. Having no capitalization feels like reading a very long sentence and slow down my speed a lot.

15 years ago, it was incredibly common to see folks texting things like "c u ltr". It made its way into instant messaging (where folks had a keyboard), but slowly disappeared once everyone had smartphones that autocorrected everything. I don't think it's strange to see that people prefer it stylistically. Text has very little in the way of showing emotions or intentions, so everything you can do to alter that is something to be used. People want to be less formal sometimes.