How do you dump general purpose, encyclopedic data into a relational database? What database schema would you use? The whole point of "triples" as a data format is that they're extremely general and extensible.
Now you need a graph for representing connections between pages, but as long as the format is consistent (as they are in templates/infoboxes) that can be done with foreign keys.
Table capital
ID,Name
123,Foo
456,Bar
Table country
Name,Capital_id,Population
Aland,123,100
Bland,456,200
> Most structured data in Wikipedia articles is in either infoboxes or tables
Most of the data in Wikidata does not end up in either Infoboxes or Tables in some Wikipedia, however, and, e.g., graph-like data such as family trees works quite poorly as a relational database; even if you don't consider qualifiers at all.
Those infoboxes get edited all the time to add new data, change data formats, etc. With a relational db, every single such edit would be a schema change. And you would have to somehow keep old schemas around for the wiki history. A triple-based format is a lot more general than that.
