It's not so much a technical gap as it is a time-on-sky gap. Telescopes big enough to make interesting pictures are relatively expensive, and cameras with a wide enough field of view to cover the sky in finite time are expensive. So to single-purpose a telescope to this kind of all-sky survey costs a lot of money. If you can piggy-back on top of an existing survey like the Sloan Digital Sky Survey, you've got it made in the shade. If you need to build two facilities to get the work done (one in the northern hemisphere, one in the south), things get more expensive in a hurry.
One possibility would be to grab the SDSS data and pop in select images from Hubble, CFHT, Keck, ESO, and other observatories. But they're gonna be hard to find in the synth. Hubble's field of view is miniscule. So's Keck and most other 10 and 8-meter class telescopes. (Subaru will be an exception once their HSC wide field imager goes online.) It'd be like finding a needle in a haystac