Ribis Because it's a totally impractical workflow. You shouldn't have to do a bunch of exporting & importing with extra files and 3rd party software just to modify one single tilemap. On top of that, you lose all reference to your other objects and tilemaps in the layout when you bring one over to Tiled for editing.
Imagine if you had to do this with Photoshop. Say you're working on something with 4 different layers but Photoshop doesn't support scaling so you have to select the layer, export it to a new file, open up a 3rd party tool, import the file, scale up the image with no reference to the other layers, export that file, go back to photoshop to import the file, then repeat for each layer. That would be completely unacceptable...yet for some reason people think it's fine here.