Jerry Holkins is a long-time D&D friend. His colleague, co-founder of Penny Arcade, Mike Krahulik had never played. So, they thought it would be fun to introduce Mike to D&D and make a podcast of it. Scott Kurtz, who was working at PA at the time joined and they got Chris Perkins (PA is in Seattle) to DM. It drew enough fans that they tried to run it as a live game at their Penny Arcade Convention (PAX). It was a hit and has grown since then.
Personally, I love the live events and those are all I watch. I don't try to keep up with all the weekly streams.
I miss the chemistry that the original group has but I still enjoy the shows. Think of it as improv through D&D but with gamers instead of professional actors. It works so well because Jerry and Mike are quick wits and Chris Perkins and Jeremy Crawford are excellent DMs.
The fan base is impressive and they have streamed live at movie theaters. WotC obviously sees a sizable market with this fandom, which I why they are backing it.
This is just the latest way they have embraced the strong base of D&D streamers.