There are sometimes that type of riders who can ride at a very high level and seem to have the skillset suited well for a certain type of races but for some reason they can't excel in them.
One such rider was Ramunas Navardauskas. Big engine, good finish, good TT-er, very strong on flat and could sometimes climb absurdly well for his weight, did well on various types of GT stages (both solo attacks and big group sprints, flat, hilly and medium mountains). You would think he had everything you can ask for to be good at cobbled classics or MSR but never did anything noticeable in those races.
Another was Simon Spilak. He won plenty of one-week races with high mountains and was a good TT-er but never performed in GTs and barely even rode them. Reportedly, he didn't feel well in hot weather.
Can you think of any other examples?
About Spilak, he probably could have finished top 10 in a rainy Giro (2013 would have been perfect for him).
I guess Lampre kinda ruined GTs for him, just look at the monster schedule the gave him as a neopro before turning 22 in 2008:
Paris-Nice for the gc (he finished 12th in his first stage race as a pro), then the cobbled classics (top 5 on the gc in De Panne and 9th in the RVV as a neopro is impressive), then the whole Ardennes classics, then the whole Giro before sending him to the Tour of Slovenia 10 days later.
That wasn't even his whole season, just the races they had him ride before he turned 22.
In the end he probably realized that he didn't have the recovery for a 3 week stage race and didn't want all the physical and mental stress that comes with riding for the gc in a gt.