Because for the vast majority of the series the writing wasn't just bad it was atrocious.
The premise of the show was a good one but it was as if the writers went out of their way to strip any potentially interesting elements out of it.
Let's start with the Maquis. It's a good idea to merge the crews since it opens up a lot of interesting avenues for storytelling down the road. The rigid professionalism of Starfleet having to assimilate a bunch of freedom fighters/terrorists in to their ranks screams drama and conflict. But no. The writers pussy out of that idea immediately and after one episode where the Maquis act like petulant children and Tuvok turns them in to fighting fit Starfleet officers the matter is all but dropped with only a few references in the next 7 years.
And what about the Delta Quadrant itself? OK, so you have this cool premise. The ship has been flung 75,000 ly from home and we're in the depths of unexplored space. Great! The writers can start from scratch and carve out a unique feel to this new Quadrant!
Yeah, like fuck..
The Delta Quadrant played out exactly the same way as the Alpha Quadrant just devoid of any of the iconic species like the Klingons and Romulans which wouldn't matter to the writers anyway because they would over the course of the series start to shoehorn more and more from the AQ in to the show until they had totally blown their wad and had a holographic Barcalay on the ship and were in constant contact with Starfleet and spent their time fighting legal battles with publishers and trying to get one over on Ferengi criminals trying to steal Seven's nanobots.

hpneutral:
And what about the Borg? Scorpion parts 1 & 2 were definitely above average (especially for Voyager) and the story came off pretty well but it is marred by being the start of the "Borg era" which would see the Borg become totally and utterly pussified.
Voyager herself went from a modern but lightly armed ship - clocking in at half the size of a Galaxy class starship and only actually outfitted with 38 torps when she left DS9 to head for the badlands - to the baddest Borg killing super ship in teh galaxy!!!1
The way Voyager cut swaths through the Borg was an insult to one of the most iconic and original races introduced to Star Trek. And it just got worse and worse with a myriad of two parters dealing with the Borg. Borg kids, Borg Refugees, Future Borg, Borg emotional breakdowns, Borg dreams, Borg tits and everything else in between. OK, the last one was welcome.
Now on to the reset button:
The writers on Voyager (read: total fucking hacks) loved to use this old staple. At the end of every episode the reset switch would be flipped so casual fans who suffered from ADHD watching at home wouldn't get confused. Yet another huge missed opportunity.
Their was no continuity at all. No matter how much damage Voyager took by the next episode you knew that we would be back to the old stock footage of the shiny Intrepid class cruiser flying through maddeningly dull DQ space. They even mocked themselves in this respect with Year of Hell.
The whole show was one huge missed opportunity. Bad writing is to blame. They didn't have the guts to take the show where its high concept premise deserved. Everything was dumbed down so that they could more easily rifle through the boxes of aborted TNG scripts they had. Ronald D. Moore joined the writing staff on Voyager and wanted to do something interesting with the show but ultimately left a few weeks later when he realized what colossal cunts Berman and Braga really were. That should tell you everything alone.
Look, Voyager had some good episodes. It even delivered a few classics. But by and large it churned out total processed shit and stuck a pretty box on it week in week out.
Is that good enough?