Why are dungeon replacements being filled by the specific Dungeon Finder queues?

Dax_Draconis

It is really annoying to queue for a dungeon to do undaunted dailies and get put in a dungeon as a replacement and usually at the last boss so you can't even complete the requirements for the daily without queuing again. The only choice is to leave and take a penalty and sit in another long queue or to help the group complete their daily and then have to sit in another long queue just to complete your own.

Why aren't replacements filled from the random dungeon queue only?

These dungeons are such a mess.