Why won't Yellowstone explode?
This is a complex question, and I cannot give you a complete answer I'd like to point out that there is a very important article published in February of this year on the subject: volcanoes are in an eruptable state very very short periods of time in the geologic timescale; the mobilization of magma is very rapid but its storage is kept near, but under the solidus (where rock melts). Surely there will be more studies on this.

Since the reservoir of Yellowstone is very large, the chances of having enough eruptable magma to cause Yellowstone to detonate is low, hence why its eruptions are on the time scale of hundreds of thousands of years (very small in terms of geologic time). 

The second thing I would like to address is "Hot Spot". The Yellowstone super volcano is supplied by the bifurcation of the Farallon slab as it subducts underneath the North American plate. This bifurcation causes the separated plate to sink displacing hot mantle upwards to the surface causing volcanism (Yellowstone). There isn't a clear definition of what scientists want to call it, often suggested to be a Plume or Hot Spot. It is not migrating. Yellowstone will eventually erupt. It is still Active.

Sorry I could not give you a better answer, but hopefully this illustrates some ideas that might explain why Yellowstone has a long eruption cycle compared to smaller volcanoes.