maybe just margin for error hugh? Nine or 10 maximums in 32 years at the Crucible suggests that it is difficult to make a 147 but I reckon throwing 8 successive trebles followed by a double, targets of not much more than a two or three square centimetres is more difficult.
Pro snooker players probably make maximums every week at home or in the club. Then again, I'm sure Phil Taylor has achieved many 9 dart finishes at home too.
If a snooker player retains good position throughout a break, itself part of the difficulty, you fancy him to pot the ball but you never really know where a dart is going to land until it has.
..and no, I've not managed either!