Why was PGM format selected to save map?

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PGM is the "lowest common denominator" image format. You can create, save, and edit a PGM file using simple ASCII text. Since PGM is such a simple format it is easy to debug a map at runtime and doesn't require a library dependency to use. The PGM format is also lossless; meaning saving your map to disk doesn't degrade its quality. This is probably why the authors originally used it. Creating a node or CLI option to do the conversion to your format of choice should be trivially easy.

If you are trouble with PGM files get familiar with the ImageMagick CLI, and the Ubuntu Eye of Gnome (eog) image viewer utility. It will make viewing, editing, and converting PGM files a snap.