The moniker we all know for the modular frame is STI. (At least until we get to the famous “divorce.”) The Para and the STI had magazine tubes that were proportioned for .45 ACP cartridges. That meant a fatter frame, to hold the fatter magazine tube. The .38 Super magazines were the same tubes, but with ribs crimped into them to reduce the interior size, and allow for proper stacking of the smaller .38 Super. Caspian used magazines proportioned for 9mm/.38 Super that weren’t as wide as the magazines of Para or STI.
But as an all-steel frame, it was heavier than the STI. The STI weighed half what the Para and Caspian did, and so it saved the weight, which meant a red-dot sight and mount didn’t make your handgun into an anvil. It didn’t hurt that since the STI frames were CNC machined, and thus everything was straight and where it was supposed to be, it was a snap to drill and tap an STI frame for a scope mount. Later, the makers offered the drilling and tapping for you, as much to save them the hassle of dealing with mis-drilled holes and customer “fixes” of same as for any other reason.
The rocket-like interest in the .40 S&W cartridge also weighed in.
The membership of the USPSA at this time was like a dysfunctional family: everyone in their faction was in competition against everyone else and theirs. And everyone wanted things to fall out a certain way: their way. The Open shooters wanted Open guns in matches designed for their guns. The stock shooters wanted no comps or dots but beyond that couldn’t agree. There were those who wanted the “old days” of single stack 1911s, and everyone agreed that there had to be a place for the “wondernines” to shoot but couldn’t agree on how to accommodate them.
This was when there was some fracturing of competition. IDPA split off, the organizers being disgusted at how far from “reality” IPSC competition had gotten. The Single Stack Society formed, to give an outlet for the guns of yore to shoot. The IDPA quickly siphoned off a cadre of shooters. The Single Stack Classic, the match the Single Stack Society organized each year, was populated mostly by the shooters who had begun with the single stack 1911.
However, the USPSA took note. Soon, there were a host of Divisions in which you could shoot: Open, Limited, Production and Revolver (although that last one took longer). In Limited, which allowed almost everything except comps and dots, shooters soon figured there was no point in shooting a .45. You see, with a magazine limit of 140mm (Open allowed 170mm) the .40 offered greater capacity than .45 did. And since capacity was king in “hoser” stages, no-one wanted to be caught at a disadvantage. The change was swift and almost universal.