> Why did they not mandate national (or at least EU-based) hosting and infra ?
They did, and they moved to block the acquisition of the local company handling it. What's unclear in the article?
They didn’t
> Currently, DigiD is partially managed by Solvinity, a company owned by a British investor
Britain is neither local nor in the EU
The "local" company is already UK owned though, so at most "European", not national or EU.
What I find strange is that the Dutch government does have its own datacenters, e.g. ODC-Noord (1), but they're still looking to outsource the hosting even after the current contract ends in 2027.
(1) https://www.odc-noord.nl/