I wanna say...
I think it actually has to do with all of the technology in the Focus vs the Fiesta surprisingly. MFT has not been the best and some has issues with the DCT. It may also be the supply and demand aspect of thing. In my area there are surprisingly a ton of used MK3 Foci but I rarely see a used Fiesta.
Edit : DAMN!!!! he beat me to it lol. but yeah I definitely think its the supply and demand aspect.