A couple things- an atmospheric block literally blocks the flow- slows it down and kinks it, giving storms more of an opportunity to track underneath and up the coast instead of progressing off to the east. When we get a classic block in the NA, it is a west based -NAO with anomalously high heights across Davis Strait/Baffin, and that is usually accompanied by a quasi stationary vortex (50-50 low) off of the Canadian Maritimes. So we have a low to our NE and we end up with HP to our N/NW (no GL low!) which is exactly what we want to keep cold air in place as storms approach from the SW. In the current pattern those HP areas tend to slide off the coast and turn the flow more easterly off the Atlantic, and getting a coastal low to track in the right spot for snow is more of a thread the needle deal. As we have seen this winter when we do have a cold trough in place over the east, the tendency is for storms to form further off shore.