A lot of it has to do with how different sitcoms are today as opposed to 30, 40, and even 50 years ago; sitcoms of yesteryears were a lot "bigger" and more "outlandish", they had a lot going for them as far as humor goes, there was a lot more character antics, physical comedy, and in the cases of fantasy-based sitcoms like
Bewitched,
I Dream of Jeannie,
The Munsters, etc, you had a LOT of outrageous and unbelievable sight gags, and in addition to shows like that, because of all of the outlandish and unbelievable things that happen, you didn't just hear laughter, you also heard other reactions, such as "Oohs" and "Ahhs", and in some cases, even great cries of astonishment (in the case of
The Munsters, there were a LOT of "WHHOOAAA!!!s", a couple of which I have in my own library).
Sitcoms today, on the other hand, are (supposedly) a lot more "sophisticated" and "intelligent", they no longer have the great wacky antics, or the boiterous sight gags that sitcoms of yesteryears had, they rely a lot more on realism, and the humor is more cereberal; because of that, those big and joyful laughs you heard on classic sitcoms would actually sound forced and un-natural on today's more sophisticated sitcoms, which is why most laugh tracks today are really droll and more subdued.
Interestingly enough, a friend of mine recently experimented with laughing up an episode of
Modern Family (he too has a large library of the classic 60s and 70s laughs like I do), while the laugh track made a world of difference, since the show is one of those "intelligent" and "sophisticated" sitcoms, he had utilize his softer/quieter chuckles for the most part.
EDIT: Actually, the whole "sophistication process" started as early as the 80s... I remember an interview with Charley Douglass' protege Carroll Pratt (who had his own Laff Box by the late 70s), saying that what was considered funny in the 50s and 60s was rather hokey in the 80s. In fact, take a listen to these:
They're both the exact same Pink Panther cartoon, however the first one was broadcasted on television in the mid-70s, and the second version is from a newer broadcast in the early 80s: notice the difference in the laughter used on both, the first sounds like an episode of
M*A*S*H, the second you can barely hear any laughter at all because of how muted and toned down it is.