Simply because it is the gold standard for data portability!
There is another thread about which genealogy program people started with ( Was Gramps your first? - #11 by adriandavey ), and many people started with another genealogy program, so importing data from other programs is useful. Similarly, you might want to interchange your data either with a friend or relative that uses another program, or a website (like Ancestry), so it is very useful if Gramps is compatible with that other software.
GEDCOM is very widely used (despite sometimes eclectic implementations of the standard) by other software, so it is really useful.
I remember the X25 standard for data comms, and for example how much more sensible it was to have email addresses the other way round (e.g. name@uk.ac.imperial) but ultimately ‘the internet’ won, and few people would attempt to promote X25 nowadays!