There's just way less to know and way less to "decide," so there's inherently a far lower barrier to entry. In Dota I have to know my hero; all allied heroes; all opposing heroes (there are a lot more in Dota than HotS). But I also have to know a lot about many, many items and what they all mean/cost/etc. In HotS those do't exist.

In Dota as I get better I also have to understand lane equilibrium, stack/pull techniques, whatever other dumb quirks, and I have to have mechanical skill regarding last-hitting/denying/aggro management, etc. In HotS there's none of this basically, or at least none that seems to matter much to me.

Plus a smaller map with faster traversal speed and fewer options for traversing it (since no BoT/mobility items/etc) is inherently "simpler" and more forgiving than a larger map where you have to make important decisions much further in advance of the actual moment of conflict.