For me it's the fact that I have long legs compared to the rest of my body (36" inside leg). I am 6'3". Therefore I can use a 400mm seatpost at full stretch and it's OK, but then the handlebars are too far down. The length of the top tube isn't a problem - it's the geometry I guess that means it is a long way down to the bars. I usually have to have a long hi-rise stem with riser bars just to get close so i don't get bachache in the aero position!! Really I guess I need an XL frame designed with a naturally very high front end (means the top tube might slope down towards the seat tube quite exaggeratedly, but that's the price to pay!). The best fit I've managed is my San Andreas to be honest.