I am curious as to why some people interpret TNG as preachy, and others don't.
Oh it's not all of TNG, not at all. Season 2 and onwards is generally just fine. I love TNG, but it's Season 1 (where Gene had the most day-to-day direct influence on the production BTW) that's the problem really.
Season 1 of TNG is infamously full of monologs about how shitty 20th century humans were (i.e. the audience is currently) and how great the 24th century people are now. And then how much a given planet's race is like the 20th century humans, and thusly how great the 24th century humans are in comparison.