Short answer would be availability of tagged data, in turn dependent on historical research, which is a laggy history of who's been funding the research.

This is changing, thankfully; check out e.g. the European Language Resources Association's catalogues, http://catalog.elra.info/ or any of the very incomplete list on wikipedia: https://en.wikipedia.org/wiki/List_of_text_corpora or in fact the various international Wikipediae themselves!

As for this particular project being English only ... itch to scratch probably ;-)