Because it's an independent employee owned cooperative from Norway, without any extern investors. It don't need to share data to make money. It's business model is different from sharing userdata.

PP

At Vivaldi Technologies AS (“Vivaldi AS”), protecting your privacy is a top priority. We strictly protect the security of any and all personal information you provide to us while using Vivaldi products and services. We do not share or sell information to any third party and we proactively protect all user data from disclosure, with the only exception being if requested by legitimate law agencies with a court order.

Tests (Webbkoll, Blacklight)

It is currently much more important to promote EU products to break the hegemony of the great US corporations. Vivaldi (Norway), along with Mullvad (Sweden) and Konqueror (Germany) are the only relevant browsers in the EU, after the disconinuated since some years UR Browser (France). As said, Vivaldi also include an inbuild Mail client and Feed reader, so are no need to use Thunderbird or other extern app.