An additional factor is that some folks, myself included, started competitive shooting before the whole moonclip mania (my words, I know they have been around longer than I've been alive, but pre 1980ish you didn't see revolvers routinely cut for them) started, consequently they trained and like speedloaders.
As an example, particularly with .38/357 guns, I find speedloaders MUCH faster to use since moonclips allow wobble and speedloaders less so. Just makes it easier to index the hole at speed if the cartridge if firmly seated.
Just my .02 but I've not really embraced moonclips, for the reason(s) above. And I can load with speedloaders faster than lots can with semi's and a magazine so...I guess it all comes down to familiarity and training. I'm thankful that the .45 AR seems to be making a comeback now with several makers factory loading them. 10 years ago you'd be lucky to EVER see a factory loaded case/box of these.