I think it is all Hasegawa's "fault! Their Egg Plane series back in the 1970s, while being met with derision and scorn by the "serious" modelers, gained almost a cult following. I confess to buying an Egg SR-71 at the time, and thought it was neat.
In the 1980s, when Bandai started doing their Gundams in the Super Deformed format, I thought it was just a passing fad. At the time the kits were really cheaply made, with hollow backsides and simplified construction. It went on a slow burn through the 1990s, but here in the 21st (Universal) Century, they have come back with a vengance. I guess Bandai figured out these things had widespread appeal, so they upped their game and are now producing good quality "SD" Gundams.
And the circle is now complete, with other manufacturers cashing in on planes, tanks, cars, and ships done in SD format. It's not my bag but I think they are a good thing, as they may entice people who would not touch a model to try one.
And they can provide a decompression build for us rabid modelers! 