Why did Mueller subpoena Bannon?

Especially when there were no subpoenas for some of the other Blight House staff? This article thinks it's because of Bannon's relationship with Cambridge Analytica (CA). Bannon is a former VP of CA. And Congress has been investigating whether CA colluded with the Russians to target people in the US for fake Russian news. They already know CA proposed a collaboration with WikiLeaks to obtain Clinton's 33,000 missing emails. Flynn was also under contract with CA and was looking for any hackers, including Russian, that could get the emails. So Bannon was likely involved in all this and hence his particular interest to Mueller.