Sketch is focussed on UI designers. Affinity is focussed on illustrators. Different markets, with some overlap. In many ways, within the Adobe suite, Sketch is closer to Fireworks than Illustrator.

I think part of the reason you're seeing a bit of an explosion in apps, in addition to the general digruntled-ness with Adobe, is that Illustrator is one of those apps that is serving many different markets — not all of them well.