Partly it was DC cancelling the title and failing to follow up Wolfman with a high profile creator.
However, I think it's a bit more complicated than that.
I would say Wolfman had grown complacent with the team and really needed a break.
He fell into a 'safe-zone' where the team had become too precious, and almost every new character he introduced would become cannon-fodder or expendable to keep them safe. The only new member that didn't die (at least not yet) was Jericho.
And Danny Chase. We can always point to Danny Chase as the Titans' 'Cousin Oliver'.
Then Jonathan Peterson took over as editor and pushed Wolfman to take chances again with the team. Some of these ideas were controversial, but it did put the Titans back on the map again with three ongoing titles being published simultaneously: New Titans, Deathstroke and Team Titans (and a fourth one, Titans West, planned).
But when Peterson left, and Wolfman continued without him, the book started to just tread water as DC just didn't know what to do with it, and gave it artists not quite on par with the best Titans or X-Men artists.
The last 30 issues is basically a ship slowly sinking and DC not making any effort to save it. Instead, by this time they were focusing on keeping their high profile creators on Batman related books.
The launch of Image was also impacting DC, as well as Marvel, as far as who to use and who to trust to stay.
Unfortunately, since the Zero Hour line-up didn't set the world on fire, DC determined that simply changing the line-up wouldn't work and they cancelled the book.
A good rundown of the sinking boat
here.
They had an opportunity with Zero Hour and the #0 issue, but it saw Wolfman continuing on with another unknown artist, and with a new team line-up that I would say rivaled Marvel's Avengers during the time when Black Knight, Sersi, Crystal and Vision were the main characters. I think I would say I would've expected a bit more for a major change in direction.
Another good rundown, focusing on the Zero Hour team, is
here.
It didn't help that when the book was finally cancelled, it was absent for over half a year.
Teen Titans #1, by Dan Jurgens, would launch eight months later.
It also didn't help that it was an entirely new team. While it did have some fans, it only lasted for 24 issues.
Which basically resulted in two more years without the original team having their own book and lessening the franchise's image.