Maybe, as I suggested above, it's a combination of transportation factors. At this stage no-one seems to be able to actually prove anything one way or the other - which always makes for good debate ;-)

Greetings, Cursuswalker,
Good question, and one that needs answering by the glaciated stone thoeorists.
I think another important question is why would the Stonehenge People, to coin a phrase, go all the way to Preseli to get stone? If they did, then there must be a reason. Otherwise, why not use the local sarsen, like they did for the trilithons? That is the key. Some people suggest the position of the Preselis in the west for the reason, and its association with that direction with life, death and ancestors. The truth is we will never be certain, as we can never ask the people who matter, nor did they leave written records.
It's all interesting stuff though. Perhaps we will never find out, which, on the positive side, is good for argument and discussion, for we can't argue with hard fact.
Regards,
TE.