He wanted to play dominant football without the risk element and his teams at club level were never coached to apply the counter press because Van Gaal didn't want his teams to play too high up the pitch in defensive transition. The new breed of manager with pressing and counter pressing capabilities would then cause him problems.
Van Gaal's teams never played football like Guardiola's Barca or the current Man City. Because for one, he never emphasised vertical plays through the lines against the press or had the counter press as the fall back option in the event the ball was turned over high up the pitch with numerous players committed in the final third. Guardiola‘s Barcelona were brilliant on the ball and played the game in the opponent's half whilst also being one of the best counter pressing teams in Europe at the time. Van Gaal didn't play like that.
And again, the recruitment being bad during his tenure was his own fault. He wrote in his book that he always had the itch to be the all powerful manager in England where he could play the role of both head coach and DoF. He got his wish and made a mess of recruitment but he also got rid of players for peanuts like Nani, Chicharito, Keane, Rafael etc, prematurely. Those players could've helped with stability and made the club much more money in sales. Michael Keane was let go of for £2m and he was then sold on by Burnley for a fee that had the potential to rise to £30m.
And Van Gaal being under the impression that United could sign any player they wanted to sign was also another reason why he failed. You can't just go to a club like Bayern Munich and sign Muller and Hummels if Bayern weren't going to allow them to leave.
Moyes, Van Gaal, Mourinho and Ole operated as managers and not head coaches. And that's one of the factors why they failed when it came to recruitment. Any prospective new man coming into the club as head coach must demand there's a DoF with a support structure in place to allow them to develop on pitch performance. None of the aforementioned wanted to operate as head coaches and hence their failure was inevitable imo.