I deliver wedding cakes to the majority of hotels in Cardiff, and while I would LOVE conventions to be held there, it is stupidly expensive
You would end up in a hotel on the outskirts of cardiff and still have to catch lengthy bus journeys in to get to the city centre. I cannot imagine the hotels in the centre considering being a convention venue. Similar reason as someone has quoted for the centre of Birmingham hotels.
Most of the city centre hotels in Cardiff are trying to get their star ratings raised, so turning into a convention hotel wouldnt give out the impression they are going for
The only real convention hotel in the centre of cardiff is the Mcdonald holland house, and to be honest, the prices for that are through the roof, they are (I think, but could be wrong) a 4 star+ (cant get any higher as they dont have any grounds)
You could have conventions in hotels like the Village, which is lovely, and very accessible being right by the entry to Junction 32 of the m4, but I think that would be expensive too, and you wouldnt gain anything as you would still need a bus journey into the centre of cardiff, or a 15-20 minute walk into Whitchurch, a tiny Village/suburb of cardiff, with really not that many amenities.
I love Cardiff, but not all flights go into Cardiff airport either, and our car parking is ridiculous.
I spent less money parking in hammersmith london and travelling by tube all day than I did getting into cardiff and spending the day parked up and shopping.
They have upgraded carparks recently with the newly opened St Davids 2, so I am hoping that parking costs will drop loads (they have for the evenings for gigs which is awesome!)
We do have the CIA though (Cardiff International arena) which is right in the centre of Cardiff, and loads of cheap hotels around it, so that is a possibility, but I would imaine that it is rather expensive to hire, and not really that big when you compare to Earls court and olympia, and obviously not suitable for an ME event as the hotels are accross the road, so it could be a collectormania style event there
That said, if SM/ME decided to hold an event in cardiff I would be over the moon