Diaper Sex is surprisingly uncommon in this fetish.
I have a few ideas why that could be…
-It ruins the illusion of "innocence".
-It's impractical. Diapers are literally underwear that is made not to come off, as opposed to boxers and panties that always come off during sex (unless you have a cumming through panties or leggings fetish).
-They're too flimsy.