I'll speculate. Companies started making these things in the late 90s, when it made sense. In 2020, it might make sense to
keep selling drives. But nobody would
start making and selling these drives today.
A few things happened around the turn of the century: USB came in, 3.5" floppies went away, and laptops started to replace desktops for more people. 3.5" was clearly not the future, but customers had lots of recent work and data on them, that they needed to access from their new computers.
The likes of Apple and Sony sold 3.5" USB drives with their new laptops (for way more than $10!). At some later point, Apple and Sony's customers had moved on, but the ecosystem of parts vendors, remaining customers, sellers etc would've been enough for the cheap generic vendors to move in.
What happened instead with 5.25"? I'd say: the market was smaller, because 10 years of growth hadn't happened yet. The market was much more fragmented, most computers/OSes couldn't read disks written on another vendor's system. There were fewer pieces of $$$$ equipment (synthesizers, industrial controllers) that embedded them. And finally, the 5.25" -> 3.5" transition was pre-USB and pre-laptop, so desktop users just bought desktops with two non-portable drives using the native disk interface.