Because that’s the point of those shows, to show that humanity has evolved beyond all that we are now, to work for the greater good and to better themselves, and I love that. Sci-fi usually covers dystopic futures and dark themes (which I love) and the fact that ST shows us an optimistic futures makes it unique, at least to me, it makes it a future that I would love to be a part of.
I also loved DS9 and ENT for the same reasons and more, DS9 is actually my favorite ST incarnation, but still the heart and soul of ST is the future that TOS and TNG presented, and that’s the same reason we can’t have ST nowadays, everything has to be dark, gritty, realistic and things like that, and a franchise like ST is completely the reverse of that. I love movies and series that are dark, gritty… and I also like when that is applied to existing franchises, only if it’s done right, respectfully and in a way which will add more to the story, what I don’t like it’s that it became the norm for EVERYTHING. Hopefully things will turn around and we will have a little bit of everything, which I think it’s the best way.