Because when I started in the early sixties compact cars had six cylinders or a 273 v8 that really was a joke. B Bodies on the other hand had 383, 413 Max wedge, and 426 Max wedge engines that produced big power and in the mid sixties Mopar installed 426 street Hemi's in the B's. When I returned from Viet Nam I bought a 383 Road Runner that turned 14:30's stock with 7" redline tires and still could carry six Marines in comfort from Camp LeJeune N.C. to Detroit on the weekend. Some of those compact cars had 340's put in them so they could try to compete with the B Bodies, but Mopar had other plans like 440 six packs and Hemi's

This was ALL manufacture stock mind you and the compact car guys only got the factory performance stuff at the end of the sixties. So to answer your question the guys in my generation went for the midsized cars with the big power rather than to play with compact cars that had little power to start with (much easier on the finances back then).

OOp's I forgot the 440's but they were boat car engines until Mopar decided to use it in the '67 GTX.