Why do so many baptisms occur with regular clothes on and not swimwear? Why do so many baptisms occur with regular clothes on, and not in a swimsuit? 

On my visit to a Southern Baptist church, I saw a little girl get baptized by immersion. That was the only time when I saw a live presentation of a baptism. I was surprised that she didn't wear her swimsuit either, because I thought the wet clothing was going to feel very uncomfortable. 

Apparently, that behavior is normal practice. I watched a couple of videos online of baptisms by immersion, and many of them involved getting your clothes all wet. When I say clothes, I mean clothes that you would normally wear casually, not for swimming.
In most churches this is more a practical thing than a theological thing. Most people don't particularly want to appear in public in swimwear, and churches certainly don't want to leave open any possibility of being seen to be parading scantily clad people for baptism. Some traditions use baptismal robes, which are white and usually worn over regular clothing.

The uncomfortableness of wet clothes is often addressed by people actually wearing a swimsuit, and casual clothes over the top. In any case, it's absolutely normal to go and get changed as soon as the baptism is over. The amount of time you are in wet clothes is very small; and since baptism is at least in part about entering into the suffering of Jesus, it's not inappropriate.

As for damaging the clothes, baptism candidates do usually get choice in what to wear: they usually make sensible clothing choices, and get advice. Cotton shirts are popular clothing, and silk and wool unpopular. Dresses and skirts are unpopular because they float up. But its really about what makes the candidate and the rest of the congregation comfortable.

If you were asking why baptisms aren't conducted naked, all of the above applies double. Without a clear and incontrovertible biblical or theological reason, the downsides of naked baptism would clearly outweigh the positives.