The actual lens quality, the overall hardware, and the use of LCoS technology vs. LCD. Both the JVC and Sony are native 4K projectors (newest JVC) as opposed to a pixel shifting or e-shift version. The previous JVC models can be had under $3,000 and are priced near the Epson 5040 model. They deliver a quality of contrast which the Epson simply can't touch and once again have a build quality which is just a fair bit better than Epson. Not that Epson does a bad job, but JVC has a long track record of very high quality and reliability. Power supplies, lens, parts and pieces. The JVC projectors weigh in about 25% heavier than Epson projectors despite similar size.
My mentality, since I have a reasonable 1080p projector right now, is that I couldn't justify spending $2,300 or so on a top of the line Epson 5040, when the JVC RS440 exists and can be found (with patience) used for $2,500 or so. In a few years we will see all the native 4K models, like the Sony 285, coming in well under $3,000. But, in a good room, Sony and JVC rule the roost.