It is well-known wikidata does not scale. Whether it is in terms of number of data contribution or number of queries. Not only that, but the current infrastructure is... not great. WBStack [0] try to tackle that but it is still much more difficult to enter the party, than it could be. Changes API? None. That means that it is not possible to keep track of changes in your own wikidata/wikibase instance improved with some domain specific knowledge. Change-request mechanic? Not even in the roadmap. Neither is it possible to query for history of changes over the triples.
Wikidata GUI can be attractive and easy to use. Still, there is big gap between the GUI and the actual RDF dump, that is, making sense of the RDF dump is big endeavor. Who else wants to remember properties by number? It might be a problem of tooling. Question: how to add a new type of object to the GUI? PHP? Sorry.
I do not downplay the role of wikimedia.
And why should it? The triples (and hence the full RDF dump as well) are a “lossy” (there's actually two different translations, the “truthy” triples that throw away large parts of the data, and the full dump that reifies the full statements, but is therefore much more verbose) translation of the actual information encoded in the graph. Revision history for the _actual_ items has been queryable via the Mediawiki API for a long time.