There are two reasons I asked, other than wanting to give the sub-levels another go.
One, even on those early dungeons there was a window that popped up when I moused over them showing what classes I used to complete the dungeon. This made me assume that I would be able to go back to them with other classes once I finished them the first time, so I was confused when they disappeared.
The second issue for me was right after the treasury was looted, I died a few times on the new dungeons. I was losing gold because I was buying preparations and it occurred to me that if I didn't beat one of the new dungeons soon I would run out of gold, barring me from bringing equipment into the dungeon and making them even tougher. Now I realize that I could exit the dungeon before I died to earn a little gold if it came down to it, but farming easier dungeons seems to be a faster way to earn gold. I guess the fact that the game told me I was leaving he tutorial section but still took those first few dungeons away for some reason left me a bit confused as to how the rest of the game would play out.