Number one reason is going to be lack of people who need to read (or write!) 5.25" disks.
But the biggest technical reason is the way USB does disk devices. For one thing, the only standard sector access by block number, and assumes a DOS filesystem. (I think digital cameras now also have a second way that is file-based.) And there were very few disk formats in use, mostly 1440K (HD) or 720K (DD). The only other common 3 1/2" format was Macintosh, with the 1440K being the same HD MFM, and with the GCR variable-speed 400K and 800K formats, and I'm not aware of any USB drives that support anything but 720K/1440K disks. And then there was the Amiga, I don't know the details, but I think it was almost but not quite MFM DD.
With a 5 1/4" drive, first of all you have to add single density, but that only covers PC disks and 8-bit disks that you can generally read with an appropriately ancient PC and no special hardware. And it doesn't deal with the 48tpi/96tpi problem. What would you do about the Apple II (GCR), Commodore 64 (a different GCR), and Atari 800 disks (MFM with the drive motor slower to get an extra sector per track). And each of them had their own completely different filesystems. And then there were flippy disks, which you can't use a double-sided dirve to read because the tracks go backwards and are about 2 tracks off.
What I would kind of like to see is some kind of USB CatWeasel that could do track reads and writes. Making low-level disk controller hardware other than using one of the standard MFM controller chips isn't exactly trivial and involves scary analog stuff. The cool part is you could do 8" drives, which I would like to do (I have an old PCI CatWeasel and some assorted drives stored away) to read some old TRS-II TRS-16 CP/M and RSTS floppies I have accumulated over the years.
Edited by Bruce Tomlin, Tue May 10, 2016 12:33 PM.