Was too much pressure on Schumacher upon his return.
Kimi had only been away from the sport for a few years but actively kept racing in other competitive Motorsports so he was always on the ball and ready for a return whereas Micheal had been away that little bit longer and had gone into full Motorsports retirement.
Not a great comeback for Schumi but to me and for many he's still the greatest F1 driver of all time.