Hoi,
At the time there were enough people who did not want a Wikipedia Foundation so it did not happen. People insisted for the Wikimedia Foundation to concentrate on Wikipedia and English Wikipedia at that and that is what happened. In a marketing driven organisation, there would be people specifically tasked with understanding, developing, optimising the other brands. It was the German chapter that developed and still develops Wikidata. Key parts are left to the Wikimedia Foundation; they are integration of Commons, hardware and performance search and marketing...
The assertion that all the other projects are there to support Wikipedia is not easy to explain. What is abundantly clear is that the existing bias for the support of English means that the market for English is largely saturated. We are at a point where Wikidata is at a point where it overtakes English Wikipedia in supporting the other projects.
* Commons is now searchable in any language thanks to Special:MediaSearch [1]. It just takes further development and marketing to make Commons bigger than many of the commercial alternatives because of this.
* Scholia needs internationalisation and localisation. Having said that, it already points to later papers for what you find in the reference section of many/most articles. It follows that what was a NPOV at a time is no longer neutral. Increasingly Scholia templates find their way on English Wikipedia articles.That is how it "serves" Wikipedia
* Scholia is increasingly used by scientists in their professional capacity. The demand for Wikidata increases autonomously as a result.
* Given that Wikidata knows about Wikisource, it could know about the status of Wikisource books et al. This provides a basis to market the finished product to an audience that would exponentially grow.
* This is not exhaustive
The point is that Wikimedia Foundation needs to market to realise its goal; share in the sum of all knowledge. That is what it is there for and has written a 2030 strategy for. It should not be beholden to Wikipedia or anyone.
Thanks,
GerardM