It's hardly SWI's lib, clpFD is available almost identically in Sicstus, Ciao, GNU, B† and others. As to why? I dunno I guess for a lot of us constraint programming just comes to hand very naturally. It often feels more Prolog-y to us than non-constraint approaches.

†B-Prolog of course morphed into Picat which is also mentioned and Picat also leans heavy on clp so if the author is more familiar with Picat it would be even more natural to use clpFD.