Another problem is the failure of these shows to create compelling new races or antagonists. TOS gave us Vulcans, Klingons and Romulans, all of which became staples the Star Trek franchise. TNG gave us Cardassians, The Borg, and Ferengi while expanding on the Romulans and Klingons. DS9 expanded on the Cardasians and Ferengi, and introduced us to some fantastic villains like Weyoun, Dukat, The Founders and The Dominion as a whole. What did Voyager and Enterprise give us? The Kazon? The Suliban? I suppose you could argue that Voyager expanded on the Borg a little bit, but it felt like they had already run their course with TNG, and Trek writers in general kept going back to The Borg too often IMO. They were no longer as interesting as they once were.
I'm going to agree with your assertion that neither Ent or Voy
introduced any noteworthy alien races. The Kazon and Suliban were both infinetly forgettable, (although I thought the Xindi were at least interesting).
But I want to knit pick some of the other stuff here. IMO, TNG introduced one, and only one, great new alien race, the Borg. The Ferengi were only interesting "per character". In other words, not interesting as a race, but some of the individual Ferengi were interesting. TNG did do a great job of exploring the Klingons.
I found nothing interesting about the (DS9) Carrdasians. They were very similar to the Romulancs, haughty, condescending, not unlike the Vulcans too.
Voy did more than "expand" on the Borg a little bit. The show got deeply into that races' psyche and "culture (as it were). I think they did a great job on this.
Enterpresie was well on it's way to as deep an exploration of the Vulcans as we'd seen, what with the revelation that the High Council had been infiltrated and Romulan spies were everywhere, when the Ent had the plug pullled. Enterprise was also poised to present the most unique take on the Mirror Universe we'd seen; that is, the Mirror Universe as a "standalone" show within a show.
Ent may not have presented any great new alien races, but the show had a style completely it's own and certainly did not lack for creativity, at least after the first two seasons. None of the other spinoffs created the pre-credits cold openings like in Storm Front pt.2, IaMD. Ent was also the first Trek series to build an entire season around one single mission.
In fact, I would venture to say that from a creative standpoint, just like TNG and DS9, Ent's star was on the rise starting with seasons 3 and 4.