Short answer would be availability of tagged data, in turn dependent on historical research, which is a laggy history of who&#x27;s been funding the research.<p>This is changing, thankfully; check out e.g. the European Language Resources Association&#x27;s catalogues,
<a href="http:&#x2F;&#x2F;catalog.elra.info&#x2F;" rel="nofollow">http:&#x2F;&#x2F;catalog.elra.info&#x2F;</a>
or any of the very incomplete list on wikipedia:
<a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;List_of_text_corpora" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;List_of_text_corpora</a>
or in fact the various international Wikipediae themselves!<p>As for this particular project being English only ... itch to scratch probably ;-)