I think his main problem at United wasn't technical or physical even if they were the final outcomes on the pitch, it's that he seemed a bit precious. He even hinted towards this, he more or less folded once he wasn't instantly accepted as being a proper United 9, or maybe because he was questioned. Rather than simply doing the business to turn that tide he seemed to respond to it poorly and lost confidence.
For whatever reason he just didn't mesh with the club. Seems somebody that needs to feel like he's the top man. He did alright for us but he was always going to be far better elsewhere, I think this was generally known and accepted so it's not a surprise. With the right setup and mindset he's a top striker, even for us and an average Everton side he scored goals, he's always going to do that, but for Chelsea he's using his other strengths far more in addition.