Austin Theory's call-up to WWE RAW was a call made by red brand Executive Director Paul Heyman, according to Wrestling Observer Radio. This was not a call made by Vince McMahon, but he is still making the final decisions.
It was noted that Theory was brought to RAW because creative needed someone to work the RAW Tag Team Titles match at WrestleMania 36 as WWE United States Champion Andrade was injured. That match saw Theory and Angel Garza lose to The Street Profits. WWE officials were impressed enough with Theory that he got the nod, but there's still no word yet on why they didn't go with someone that was already on the main roster.
As noted last night, WWE moved forward with Zelina Vega's new stable that includes Andrade, Theory and Garza. Despite the prominent roles for Garza and Theory, they are still listed as members of the NXT roster as of this writing, and are not on the RAW roster. WWE officials also reportedly liked the idea of Vega leading the stable of Theory, Garza and Andrade.