Not many clubs were interested in Onana, and even if there are, almost none will make an offer to Man United. For example, Inter Milan is rumored to be planning a reunion with Onana, but until the transfer window ended, Inter Milan never made an offer. I think Trabzonspor was the only club that made an offer to Man United, and it happened right before the transfer window closed, so everything had to be done quickly.
It's quite reasonable why not many clubs are interested because Onana's performance has been quite bad so far and he is known for making frequent blunders.
Because he was only placed on the market after they got his replacement on deadline day giving Europe top 5 league clubs no chance to get him. So it's unfair to say no top clubs came in for him and how many top clubs are without goalkeepers?
He was bad, he had a poor season but we should also remember how he came in, he came to united as one of the best goalkeeper in Europe so there's something absolutely wrong at United. He received the most shots on goal last season, that alone should tell you how bad the defense was. He was exposed most of the time, no defense protection.