Suppose you have one staff in 3/4 and another in 4/4. And you want a double bar line at the end of the second bar of the 4/4 staff. That won’t line up with a bar boundary in the 3/4 staff. If add a double bar anyway, the double bar line goes through all the systems, including the 3/4 staff not at a bar boundary.
If you want a double bar line in JUST the 4/4 staff (and leave the 3/4 staff alone), how would you do that?