Why don't they use time travel more?
I'm going to leave aside the Temporal Prime Directive, because the answer to this question is really about the motivations behind that regulation:
The consequences are incredibly hard to predict1
Enterprise probably explores the consequences of time travel more than any other Star Trek series, and we get a wonderful example of this in the season 1 finale/season 2 premiere Shockwave. At one point in this episode, Captain Archer gets transported through time from the 22nd century to the 31st. As a result of him not being in a specific place at a specific time, the 31st century looks like this:
Needless to say, it's not meant to look like that2. This is a bit of an extreme situation, given that there were a lot of people futzing with time while this is going on, but ultimately all of the tight regulation of time travel boils down to this reason.
Voyager actually has a whole two-parter that reeks of this problem: Year of Hell. The main antagonist is a scientist who's devised a way of mapping changes to the timeline, and calculating the effect that interventions will have. Using this, he's spent two hundred years trying to restore his civilization to glory and prevent the death of his wife by performing surgical strikes against planets at different points in time.
At no point does he succeed; every single attempt to restore his desired timeline causes another problem that he needs to solve. It's implied by the end of the episode that his problem isn't solvable, although it actually sort of is: fixing time so that the ship never existed.
The point of all this is that time travel should only be considered a viable solution to a problem if that problem is of significant importance to the Federation, or if the problem was caused by time travel in the first place. Unfortunately for Worf, the death of his wife doesn't qualify.
Why wouldn't someone try anyway?
There's not a lot Starfleet could do to stop an individual using time travel without authorization. This is a fair point, although there are some counterpoints.
For one thing, we've seen evidence of Federation-aligned organizations working to preserve the timeline. The DS9 episode Trials and Tribble-ations introduces us to the Department of Temporal Investigations, a branch of the Federation government tasked with following up on time travel incidents.
Two separate Voyager episodes introduce us to the idea of Starfleet policing the timeline in the 29th century: Future's End, which shows a 29th-century timeship ready to destroy Voyager to prevent a temporal incursion in the future; and Relativity, which has a similar timeship recruit Seven of Nine to prevent disruptions to the timeline; finally, a major plot of the Enterprise series involved Daniels, a Starfleet Temporal Agent from the 31st century, trying to correct various attempted corruptions to the timeline.
Although these are the only occasions we see these organizations on-screen, we have to suspend our disbelief somewhat and assume that they kept working even when we don't see them. So any individual attempting time travel for their own ends (Especially one within Starfleet) would presumably have to answer to these organizations eventually.
My other counterpoint is more wishy-washy, but no less valid. Everyone in Starfleet knows the dangers of time travel by the time of DS9. In fact, every time there's a time travel episode we get at least a half-dozen lines of dialogue reminding everyone that our heroes can't do anything that may affect the timeline. So they know that there are regulations against it, they know the logic behind the regulation, and they agree that the regulations are sensible.
In the 21st century, you'd expect someone to give it a shot anyway. But in the world of Star Trek, that's usually been enough. TNG actually gives us that message as a minor plot point: The Neutral Zone gives us three humans from the 20th century who have been cryogenically frozen, and a subplot of the episode is them trying to adjust to the culture of the 24th century. In particular we have this (Paraphrased) exchange after one of them uses the comm to summon Picard:
Picard: You're not authorized to use that.
20th century investment banker: Then why isn't it guarded?
Picard: Because most people have enough sense that it's not necessary.
Needless to say this utopian vision of Star Trek softened somewhat after Gene Roddenberry's death, but the fact remains that most citizens of the Federation, and Starfleet personnel in particular, are sensible enough to know that the consequences outweigh the benefits of trying to use time travel this way. And for the ones who aren't, that's what the trigger-happy 29th century timeships are for.
1 There's some real-world basis to this, called the butterfly effect (good name for a movie). Essentially, dynamic systems, like the weather, are extremely sensitive to initial conditions. That's why you sometimes hear the adage "a butterfly flapping its wings in Brazil causes a tornado in Texas"; small changes in initial conditions (the atmospheric disturbances caused by the butterfly) are amplified by the interactions of the dynamic system, causing disproportionally huge results (tornado). Now imagine time as a dynamic system...
2 As Eric Smith points out in comments, the use of the word "meant" is somewhat troubling; it implies a measure of predestination which is difficult to justify philosophically. When I say "this wasn't meant to happen" I mean "to the best of our knowledge2, the only way this could have happened is by the direct intervention of time travellers". We can say this in the Star Trek universe, because it's been established that there is only one timeline, which can be changed from right under your feet. The City on the Edge of Forever, Yesterday's Enterprise, and Year of Hell are just three episodes that spring to mind that reinforce this4.
2 The stipulation "To the best of our knowledge" is important because, as Eric Smith also points out in comments, it's possible that history as we know it was caused by time travellers, and the timeline-detecting technology employed by Daniels and crew aren't sophisticated enough to detect this. We definitionally have to exclude this from consideration, because we have no way to prove this interference. If we've decided that meddling with the past is Bad (And the existence of Daniels indicates that we have), all we can do is police the incursions we can detect.
4 As Eric reminds me again, we have to rectify this "single timeline" against the existence of things like the Mirror Universe. This is a hard thing to do; every episode about time travel we've seen indicates that there's only a single timeline, and yet it's also a many-worlds situation where Zefram Cochrane blows away a Vulcan landing crew and founds the Space Nazis5. I'll leave the mechanics of reconciling this to the philosophers, and just note that Our Time Travel is Different
5 Not to be confused with Nazis....IN SPACE!
