I think one problem is that it's really hard to do structured data in general. Projects that pick a specific domain tend to do it much better, because they have a more tractable problem, can build a community with domain expertise, etc., in ways that Wikipedia will have trouble matching unless they plan to collaborate with those projects and/or pull data from them. For example, I think a structured-data version of Wikipedia artist/album infoboxes is going to have a long way to go to catch up to http://musicbrainz.org/, which has a carefully thought out ontology and years of iteration on that specific problem. Alternatively you can try to do a carefully thought out, consistent schema for all metadata, but the Cyc project shows how hard that is.

I do think that by virtue of breadth Wikipedia's version may become the best data resource in niches that have no specialized structured-data project for them, and it may give other informal-schema, broad-coverage projects like ConceptNet a competitor.