I'd argue the .38 Super was never truly mainstream at any point in it's life. In a 1911, it's overshadowed by the ubiquitous .45 ACP, and to an extent the muscle of the 10mm Auto. To CDW4ME's point, "bigger is better" permeates so much of people's thinking. I think people see the .38 Super on a 1911 and feel the same way they do about a V-6 Mustang; "Oh, cute, where's the rest of it?"
That said, I have a stainless Colt Competition in .38 Super that I just absolutely LOVE to shoot. It's very mild in recoil, flat shooting and definitely gets people's attention at the range. Mine is fairly accurate and not at all picky about magazines. Any 9 or 10 round magazine I've found feeds 100%, though I have one magazine that doesn't like to lock the slide back on empty. (And it's been a bit fussy about what bullet profiles will feed 100%.)
Ammo is far less common than for the .45 ACP or 10mm Auto, no question. Magazine capacity and ammo choices are limited compared to 9mm Luger and .40 S&W. It's out there, just not sitting on every ammo counter shelf.
But if you just like the cartridge, just want something different, and understand what you're getting going into it, it's a lot of fun. I look at my .38 Super as a 9x19mm +P that can handle 130-147 grain bullets with ease.
(Yet after all of that, I also have a 1911 in .45 ACP due to the much bigger selection of JHP ammo, and having an affinity for big-bore handgun rounds. My particular .45 has even proven to be less fussy about what bullet shapes it will feed.)