Think it's came down to the fact that Red Bull have too many drivers now.
With Vettel gone, that left a promotion to one of their drivers, they could only promote one of Kvyat or Vergne, by taking either then the two Red Bull seats would be covered for a several years, just like it happened with Vettel and Webber.
The two Toro Rosso drivers are now looking good, especially the one no one gave much a thought about Carlos Sainz, but we all know Verstappen will get the Red Bull seat ahead of him no matter what.
They put themselves in a little dilemma as the Verstappens said they would only join them if he got an F1 seat this year, so they did, just to get him away from Mercedes.
If we look back last year, Kvyat was a bit like Verstappen, 19 years of age, fast, but with a little less hype.