Answer: Because of corruption! It's illegal to use Starlink in Greenland, and Tusass holds a concessioned monopoly over “telecommunications services in, to and from Greenland” and the underlying infrastructure! https://www.aqutsisut.gl/en/tele/satellite-regulation
Tusass was in talks with Starlink to basically provide Starlink service but via the Tusass monopoly, basically making Tusass a no-value-added reseller of Starlink at massively inflated prices (subsidized of course, basically being paid by the Danish government to do nothing besides cash checks while Starlink does everything else). This is so obviously corrupt that it's better for them to use a worse, more expensive service that doesn't make Tusass completely pointless.