Because they work better, have a greater endurance, carry more powerful weapons, and other inconsequential stuff like that.
Bill
Now, THAT is one admittedly VERY simplistic way to look at things. To say that nuclear boats "work better" is just wrong.
Ask any nuc driver what he fears most. Diesel-electric subs of German origin will likely be at the very top of the list. If there's a German crew manning the sub, most even admit they'd rather be elsewhere than in the same area.
A couple of years back, a German 206A managed to penetrate the protective screen around a USN carrier goup, including tow LA-class boats and "sank" the carrier without the escorts being able to locate the boat. Interestingly, it wasn't the first time this had happened in an exercise. Even in the case the boat had been "sunk". Compare the cost of one 30-year-old sub with the cost of one supercarrier and its assorted planes. I won't even talk about the crew size. 24(?) versus 5.500(?)...you do the math. The German-made boats (especially the 206A) are too small for the US sonar systems to be picked up by active sonar. They are simply filtered out by the program. In a sub-on-sub scenario with a nuc and a d-e boat,, both with halfway competent crews, the non-nuc will almost always emerge victorious. They're more maneuverable and much quieter.
Nuclear subs and diesel-electric subs do have completely disparate tasks. Nucs are pure blue-water boats used for defense of carrier task groups, hunting other nucs, the occasional strike mission with cruise missiles and disrupting enemy shipping. They are fast, have high endurance but they're also patently unable to bottom the boat for longer periods (the reactor cooling pumps tend to chocke on the silt) and they're comparatively loud (and never mind the Clancy-esque propaganda about American subs being the quietest subs around...it's just that: propagandistic claptrap). At speeds greater than about 26 knots they sound like a gravel truck. I heard one myself on passive sonar, 2005 in the Med. The sound signature of the reactor pumps is another Achilles' heel.
"Conventional subs" specialize in littoral warfare. They're small, extremely quiet, pack an awesome punch in relation to their size and close to the coast can do anything a nuclear boat can do, only much much better because their Achilles' heel, speed and endurance, doesn't factor into the equation as much in this case.
As a rule, the further a nuclear boat comes inshore, the more the disadvantages outweigh the nuc's inherent advantages. For conventional boats, of course, it's vice versa.
And the US has been trying to get their greedy fingers on German submarine technology for years. They even tried to buy HDW shipbuilding through a Spanish investment bank . Luckily, they failed. I think this goes to show how much nuclear boats "work better" than coventional ones, right? My ass...