Opening of the market- gaming was suddenly less important then a consumeable story. Everyone can consume a actionflick- not everyone can become a general and really keep a battlefield coordinated. The genre is also not so ideal to tell a story.
All of the other team based MOBA/FPS games seem so much more accessible and immediately rewarding. Aside from the current free-to-play boom, I think big companies like Blizzard see this and pour resources into Overwatch and Heroes of the Storm.
As a person who has played starcraft regularly since BW and SC2 beta and still plays today, if I was a kid looking at the competitive gaming landscape today, i feel like there's little chances I would go for and RTS over everything else that's out there.