Raonic is tall-ish, but /u/YaoPau has noted players like Paire who are similarly tall do not have as a good a serve as Raonic. It is a factor, to be sure, but he is much shorter than Isner, Anderson, Karlovic, and Janowicz, while having at least as good a serve, and possibly better.

These taller players generally do opt for hitting bigger shots because usually their movement suffers. Players that move better (say, Cilic) tend to have steadier groundstrokes.

What's hurt Raonic has generally been his return game, and that's mostly been because he holds so often, he hasn't needed a great return game. I tend to view these things as players having certain skills. When you have a big serve, you don't need a big return to be successful. Similarly, if you're serve is somewhat weak, you better return well, or you won't get to the top of the ranks.

Even players like Karlovic and Isner who are very tall typically have very good forehands. It's too hard to play good tennis with only a big serve. You generally need a big serve, and a big forehand. You can get away with an average backhand and average movement with those two strengths.

Raonic has generally had a powerful forehand, but has struggled to keep the shot in play, and a little with his movement. He's working on that, plus his return game, because if his return ever gets decent, he'll be very hard to beat. Right now, players like Nishikori or Nadal tend still have very good groundies that can put Raonic a bit on the defensive when he's returning. Other players tend to have better serves which Raonic struggles to get back.

I think it's just a little easier for Raonic to build a better return than it is for Nishikori to build a big serve. Hardly anyone improves their serve by that much. The only guy that kinda did it was del Potro but he happened to have a weak serve for his height. Nadal briefly improved his serve pace, but I think it was causing pain in his shoulder, so he decided against that service motion. Murray has improved the pace of his serve, but he doesn't serve a high percentage of first serves.