Why isn't it? I think it was a series of progression (no pun intended). I truly feel like we consistency got progressively stronger stories the longer Trek was going on. I also feel like we (collectively as fans) also got collectively more critical too, and on many points some of criticism was very fair and on some points it wasn't. Watching the series for tried and true progression, Enterprise was better then Voyager which was better then DS9 which was better then TNG which was better TOS. They all have their memorable points. They all have things we love about each series. They all have things we hate about the series. I won't deny that there are prejudices each of us have that will elevate one series above another, but really, I think its hard to objectively say strictly from a technical stand point, the newer the series, the better it is with one important caveat: Robert Picardo made a very good point in "The Captains (2011)" when he said no Star Trek series really got good until mid way through season 3; that was when each series really started to find itself. TNG, DS9, Voyager and Enterprise all have that one episode right around season 3 where everything all of a sudden seems to finally work. Honestly, after hearing that and really seeing it in action after thinking long and hard about it, I've always wondered how much better Star Trek TOS may have been after season three if it would have gotten four more seasons. Heck, I'll admit, I keep wondering how much better Enterprise would have been. Season 4 was really, truly, the best Star Trek ever seen on screen IMHO. I dare dream how cool seeing the Romulan War on screen and the formation of the Coalition of Planets would have been.