One popular view on why there is no 2014 Mini quad is that Apple is trying to protect the iMac. But I don't think that makes sense considering that they introduced the 1.4 GHz 2014 model.
On the quad side, you can get a 21.5" 4K Retina iMac with a 3.3 Ghz quad for $1699. If they sold a Mini for $999 with 2.6Ghz quad with a lesser iGPU and say charging $100 or $200 for an upgrade to better iGPU, then I think the iMac would be able to effectively protect it's turf. You also can't get 5K, even with Skylake, which the 27" iMacs have.
But the low-end iMac, a 1.6Ghz computer for $1099, has nothing special to offer vs. the low-end Mini at $599 (with the 8GB RAM option). So for $500, you're get .2GHz extra speed and a non-Retina 1920x1080 monitor when there are a lot of cheap 1920x1080 monitors you can pair up with the Mini. Add $100 to the Mini and you get a Mini that'll be quite a bit faster (2.6Ghz) than the base iMac. To me, the case for the Mini vs. iMac is much more compelling at the low end and than it is at the high-end.
Perhaps Apple thought that they could force people into the iMac by not offering a quad. But I don't see how they could have thought this given the monitor options they offer in the iMac. I mean there's a lot of people who just don't want the 4K 21.5" Retina or aren't willing to spend the money for the 5K Retina. If they thought they could get people to buy the high-end iMacs by not offering the Mini quad, they certainly know now, given the healthy used 2012 Mini quad market, that they were mistaken. Instead of enriching themselves, Apple has enriched the owners and the middlemen who have sold the 2012 quad Mini's.