1. It's super tricky to get the queries right. SPARQL is so unknown by the data community, so it's hard to get help. Plus there are super sudden quirks that lead to erroneous results (e.g. not all relationships are shown if you filter the data _some way_ but no in a _slightly different way_). 2. Wikidata facts are often _not_ leveraged by Wikipedia itself. This leads to divergence between the two datasets, sadly the more popular one is also the one that's unstructured. Unless we properly tie these two services, we won't get an up-to-date linked database.
I have worked fairly hard on updating Wikidata with local facts (politics, mostly) over the past year or so and the two issues noted above have consistently dampened by motivation to continue.