I never really could figure out why 10-24 just wasn't that popular. My industrial supply in Tennessee had around 10,000 SF dedicated to fastener warehousing for Rockford®, Unbrako®, Allen® and a hand full of the cheaper brands. Millions of fasteners and not a single 10-24 stocked, SHCS, FHCS, SSS, RHMS, etc and notta single 24TPI in anything unless special ordered and then it had to be case quantity.
Fast forward to now and we've been doing the finishing machine work on the facial recognition camera housings for Walmart and some genius decided the 4 holes that close the camera housings are 10-24 x 3/4 SHCS yet the mounting pads for the electronic boards are 4mm. Talk about a head scratcher! I finally asked one of the engineers why and they said per thousand the 10-24 were about $.20 cents cheaper verses the 10-32. At first I just thought cheap bean counters then realized theres over 11,000 Walmarts world wide and each store gets around 100 cameras so thats 4.4+ million screws. Who would have thought an electronics engineer would think to price screws by thread pitch.