I think it has to do with a lot of the aspects you mentioned. Player safety, as well as the increased political aspects of the sport, has led to less interest and it's interesting to hear people who have boycotted tell you why. For every person boycotting because they don't feel the NFL is looking out for their players, there is a fan who is disappointed in the lack of physicality on the field. For every fan boycotting because Colin Kaepernick isn't on a roster, there is a fan boycotting because they want the players standing for the national anthem.

The NFL has gotten stuck on both sides of a number of issues. They need to make firm judgements about how to approach the future, and they need to do it with their true fans in mind. I think this is exactly what the XFL is aiming to do, and why they seized on this opportunity.