The writing is really bad.
This is the biggest problem. Voyager regularly deals up truly horrendous episodes at a pace roughly equivalent to the Original Series, but it doesn’t surround those episodes with the highs of TOS (which produced some of the best episodes of television ever made).
The acting on the show is also incredibly problematic. There are several performers who are just flat-out terrible. Others are crippled by the bad writing. The cast notably lacks the stellar talents like Shatner and Stewart: The best actor on the show is Robert Picardo, but it’s really difficult to run a show out of sickbay. (Which is why most of the series’ best moments come after the Doctor gets a portable holo-emitter and Jeri Ryan joins the cast.)
Coming back to the writing, though, we can also note that the writer’s room was burned out: The same basic team had produced hundreds of episodes of Star Trek at this point and they were just running out of ideas. There’s a lot of rehashed Trek fan-fiction taking the place of original science fiction ideas. (And if you peek behind the scenes, you’ll discover a surprisingly large number of rejected scripts from other series getting dumped into Voyager.)
Finally, the show embraced ideologies that were curiously antithetical to a lot of the futurism that the franchise had previously fanfared. For example, “Measure of a Man” is one of the most celebrated episodes in all of Trek, so it was weird to see so many episodes of Voyager endorsing Janeway’s position that the Doctor (and other forms of artificial life) weren’t actually sentient beings. Voyager is also where the Prime Directive reached bat-shit insanity.
What might have saved the show would have been to embrace the long-running story arc with meaningful continuity that its premise inherently promised. But meddling from above repeatedly prevented that from happening.
Nail in the coffin: The entire series hinges on Voyager being stranded in the Delta Quadrant. The writers accomplished that by making Janeway an asshole; misinterpreting the Prime Directive; and then executing a plan that makes no sense. (Put your bombs on a timer!) The entire series got off on the wrong foot and was based, ultimately, on some really stupid writing.