I always thought that Holland's M.O. was letting the prospects "over-ripen", as in keeping them out of the NHL longer rather than pushing them onto the team immediately.
And there's a consideration too about balancing what is best for the player long-term vs what is best for the organization short-term. Holloway might be better than a lot of what we already have on the roster, but we also want to make sure that he develops in a way that makes good on his potential, without having him cut corners just to be able to tread water in the league as a teenager.