There are lots of reasons why you don't see the Big Advents of yore. Part of it, as James pointed out, is that they're.....big. In addition, advancements in the theory of bass reflex tuning (thanks to Thiele and Small) allow smaller woofers (particularly 6.5" drivers) to go as low or lower than the big sealed or poorly executed ported boxes did back in the '60's into the '80's. The smaller drivers are also better than they used to be. Also, and more recently, designers are placing much more emphasis on smooth off axis response than was the case when Henry Kloss was mating a 10" woofer with a tweeter in a 2-way. That generally means using smaller woofers with better dispersion (along with wave guides for the tweeters in some instances). Finally, subwoofers are becoming more and more the norm, so there is much less need for a full range main speaker with a big hog woofer.
One thing to keep in mind, however, is that just doubling or tripling up on smaller woofers doesn't give you deeper bass. The F3 will be the same as if you used one of the woofers. What that does give you is greater sensitivity and lower distortion for a given output level. That's particularly important in HT, and the lack of deep bass can be fixed with a sub(s).
