Why 回 is used in 回事 instead of 件事?
There are only a few phrases made up from 回事:

怎么(一)回事: what the matter with ..., why ..., what is the reason for ..., what happened

这回事: The matter you just mentioned (E.g 没这回事。 There was no such thing)

一回事: The same. (E.g. A和B是一回事。 A and B are the same (matter).)

In the above situations, "件" cannot be used.