Tilemaps vs one big PNG?

I've been using LDtk and tilemaps for level building in a somewhat large map. I need tilemaps for a few things but I just tested the memory usage and performance of exporting the entire map (just the parts that don't need to be layered) into one big .png which then ends up as a .pdi

The png I used to test is about 50 MB and 12000x9000 pixels which then becomes about 2MB as a pdi

I loaded this in and made a sprite with it and even with the drawOffset always changing and the whole screen redrawing I did not notice that much of a drop in performance or increase in collectgarbage("count"). The memory used for the pdi is much smaller than the JSON and still smaller than the pdz file when I do JSON->Lua table.

Other than the slow initial loading of the image is there a reason I shouldn't do this? It seems like an easy way to lower download size and put more level loading upfront one time for tiles that are static and in the background.