Time trials are really about yourself and your preparation whereas mass start races are about your interplay with someone else. Both types of racing involve strategic thinking and planning but mass start stuff is more off the cuff and dependant on what the other guy does. Thus, I think aside from physical differences, there are big difference in the mental aspects- what people find fun or interesting.
A TT requires a lot of patience and discipline because the best way to parcel out your power is the opposite of how you'd naturally do it- you should hold back early on when you're feeling great and ramp it up when at the end when you're already suffering. In a TT you don't get the adrenaline spike prompting you to ramp up your effort in response to someone else, so you have to figure out a way to get your brain to do it anyway. To some extent, that comes naturally to some people, but it's also a matter of practice.
I think like a lot of things in cycling, two different people might start with small physiologic differences- my friend is sprinty but I am time trialy. She loves sprinting, so she does it a lot and enters lots of crits and gets better at what she's good at. But doesn't spend as much time perhaps on her TT bike and isn't as comfortable with the postion and bike handling. I love the long sustained threshold effort, so I do ride those efforts all the time and enter mostly TTs. But maybe I don't spend as much time on fast group rides so I'm not as comfortable/good at handling my road bike in traffic. It doesn't take too long for my TT skills to become way better and her mass start stuff to really become honed. Our initial physiologic differences become better trained and eventually we are metabolically quite different during race efforts.
I imagine it's like that for pros too. If they start out strongly better at one aspect of cycling and they are trying to make their way in the world of racing, they can become very impressive at the stuff they're good at. Then they have to go back and work the stuff that's less natural to them. But they have to make themselves do it, like the rest of us.
I also was just speculating to a friend today- if you want to be good at TTs only, is a sprint a harmful thing?