Because there is no need not to.

Email is ancient; it still works; it usually does not need round CSS corners nor bold HTML stuff.

"Modern" clients gives you utter shit like WebTV and midi-file signatures.

Because they are being used? (In turn because they tend to be the best ones around, most of the newer email software tends to be crap if you go by productivity rather than shininess.)

Also, this is not just about compatibility with old clients, but also about compatibility with old emails. If you redefine the meaning of previously used email syntax, that changes how old emails get displayed, which sucks if you have an email archive that goes back more than a decade.

Gmail doesn't support format=flowed, so from that perspective, it too is an old client. Last time I checked, one of the most widely used non-web clients also didn't support it (Outlook).

But the good thing about format=flowed is that even if a client doesn't support it for display, it'll still look good (on desktops)