Did you notice that the recent Happy Days reunion special was a big hit, while the almost-as-recent Dallas reunion was a ratings disappointment? There are various factors that account for this (time slots, for one). But it seems like for the most part, the shows that do well in these reunion specials, the shows that are still popular enough to draw an audience 30 years later, tend to be sitcoms.
Sitcoms are not usually considered the best TV has to offer; except perhaps in the '70s, when TV drama was going through a rough patch and the experimentation was mostly happening at Norman Lear and MTM, the critical favorites are usually dramas or sketch comedies. But dramas date fast, and so do sketch comedies. Sitcoms, on the other hand, are like roaches: the TV universe may go kaput around them, but they will survive. Most of the shows that remain popular in syndication for decades are sitcoms, apart from Star Trek and a few others.
Part of the reason for this is that comedy acting (as opposed to sketch comedy performance) and comedy storytelling dates less quickly than almost anything else. What seems like serious dramatic performance and great dramatic dialogue today will come to seem silly in a few years; but a performance that seems funny today will usually seem funny a decade later. And since sitcoms all follow the same rules (often century-old rules) of how to construct a story, they don't date as fast as comedy sketches, because there's a lot more turnover when it comes to the rules of how to construct a comedy sketch.
The upshot of all this is that even a cheesy sitcom will often last longer and "hold up" better than a quality drama. Look at Gilligan's Island. It's cheesy, stupid and predictable. But the one thing it isn't is dated; in terms of performance, story construction and jokes, it doesn't feel like a product of another time, or indeed any time at all. And audience reaction to it never changes; in 1966 it looked just as cheesy, stupid and predictable as it looks today, and was popular in spite of all these things. Moving up many notches in quality, a show like All in the Family may include dated references, but it doesn't feel dated in terms of performance and dialogue the way issue-oriented dramas do, and it remains popular where most dramas have become museum pieces.
Which, of course, is why the sitcom will never die: production companies can't afford to give up on them. A drama can be a hit in the here and now; but a hit sitcom can be a money-maker for decades after most of the dramas have been forgotten.