There are 3 possible reasons:
1.If you have already opened both pre-1.32 and the current version of World Editor, then it's obvious. You can't use both at the same time due to the fact it uses a different "Tool Window" item.
However, this can be fixed by a small edit on Registry Editor (delete the "Tool Window" item under "HKEY_CURRENT_USER/Software/Blizzard/WorldEdit, or you can use this:
This is a simple utility bat file that cleans up the editor registry so user can open their old editors after accidentally uses the newer ones. Most notable when switching from 1.32 to 1.31, or 1.32 to pre-1.31, or 1.31 to pre-1.31. The bat file can be inspected if people care about potential...
www.hiveworkshop.com
2.You can't open the maps that was saved under Reforged with the pre-1.32 ones because most of the features added in Reforged will be added.
But, you can use this method to make Reforged maps openable. Make sure you have a backup files before doing this (I've never tested it):
Have you ever wanted to play a map but couldn't, because it was made with Reforged's World Editor? Well today is your lucky day, because now you can make those maps compatible with patch 1.31 (1.30, 1.29, and 1.28 also supported, or any patch for which you have the game files since v1.0.0)...
www.hiveworkshop.com
3.Another possible reason is that it could be related to the 1.33 problems. Latest patches of Reforged tend to break some important features in WE.