The point is that IRC is short for Internet Relay Chat, not file sharing. mIRC, and other IRC programs were primarily written to be used for chatting.
The fact that you can do file transfers with these programs is an additonal option that the programmers put in. They didn't have to.
As to your query about the queues, it would depend on what was written into the code that the bot that's sending you the file. there's no way for mIRC to be able to tell how many files are being sent at one time, as they are usually sent via a DCC connection, which, realistically works the same way as a private chat. There's no way for mIRC to determine how many private chat windows someone else has open.