I'm guessing the reason is that they don't want the development to become a 'rat run' for traffic from Creigau, Groes Faen, Radyr, Fairwater etc getting on to the M4. It is understandable given their plans for avenues and streets.
One option would be to have a new link road but that would split the development in two. It would compromise what, at least on paper, looks like a well thought out 'village'.
So a case of aesthetics before practicality. Why not have a spur of llantrisant road through the centre, linking to j33? This could be developed as a linear commercial centre with retail, commercial and offices, so no residential fronting the main road. The more I think about it the less I like the proposal actually. It has been planned in isolation and not taking into account the knock on effect this development would have.1500 houses will generate additional traffic on llantrisant road. By blocking access to j33 persimmon are saying we don't want outsiders driving through our development, but don't mind forcing traffic from our development onto an already overcapacity road through other existing developments.
I don't see why they couldn't have done what they've done at junction 30 with the other end of the A4232. Build a dual carriageway from junction 33 to Llantrisant road and then build the village to the west of it. That way it's there for future developments, provides access to the village and better access to Creigiau and Pentyrch. As it stands There's still pretty shit access to the M4 for those villages, which is pretty daft when there's a junction right next to them.
Pontprennau may be shit in some respects but at least it's very accessible by car and likewise, because of previous planning, so will St Edeyrn
The "Echo" has missed this point completely, its report stating there will be access at Junction 33. I agree with George this looks like a form of nimbyism. Still, it will be interesting to hear the complaints from the new residents when they are stuck in gridlock.