Because (at least on lemmy) they spam articles about this, and treat it like some "gotcha" moment that would make the company no longer able to fulfill their purpose.
One of the owners investing in a political party should not be this big of a deal, and the articles provided are clearly made to put suspicion on not only Mullvad but also on ÖP.
The campaign is actually more about positioning ÖP as some sort of "far-right"-party by the now left, because they are afraid to lose votes to them. They also of course wants to make it "shameable" for people to donate to this party so they cannot possibly win some votes this election.