Lukaku was never the right fit for Ole in so many ways. He just doesn't suit the fluent style of play, for all the flicks and close control Rashford, Martial and Greenwood can do, Lukaku would have lost the ball in those tight spaces.
His build up play was never his strong point, it would have frustrated Ole and his team mates with miss controlled balls or miss passes.
It is clear that Ole wants to play a passing game, building from the back, passing out from the keeper and hold possession of the ball until there's a clear chance or opportunity to shoot or cross, rather than blindly crossing against a full back. And having an interchanging front 3 allows him to play that system, if Martial receives the ball outside of the ball, Rashford or Greenwood would go into the box and vice versa. But Lukaku being very one dimensional, he just didn't fit.
But the most important reason was his attitude, once being dropped he just dropped his head and threw his toys out the pram, and wanted out. Ole understands the United cultural and demands the same mentality from his players. Just look at Matic, everyone thought he was on his way out of United last season but he kept going quietly about business and eventually made his way back into the side and has been amazing since. We never saw the same from Lakaku, from all the talk of himself being a winner and has a strong mentality, it was just never the case. I'm glad he's gone.