It seems perfectly possible to adapt them for on-street running, and entirely reasonable not to do so at this point as it’s not needed yet.
But – pure speculation here – how much on-street running is actually needed in Cardiff Bay? From the concept drawings, there is no on-street running along any roads at all, only two crossings (Lloyd George Avenue, near the current station, and Pierhead St, near the A4232 roundabout). There’s plenty of room in both locations to take the line over the road in a grade separated crossing, if you wanted to (bear in mind the whole area is being redeveloped anyway). And I don’t think it would be a huge problem to sever Lloyd George Avenue at the bottom anyway (just provide a pedestrian/cycle crossing, and the cars can go round Hemingway Road). The rest of the line to Tremorfa is obviously all separate already, so for the cost of one (maybe two) bridges, you could maintain complete separation from the road network, and not bother with changing the wheels at all (north west corridor notwithstanding). Would it be worth the effort/expense?