Because of the distance from the sun

Venus's albedo is so high that the insolation at the surface is even less than Earth's. Yet it's hotter than Mercury, which is closer to the sun than Venus.

The article says that volcanism is the reason, and that solar heating would not cause this result on its own, even though it's everyone's first guess.

From TFA: "The sun alone cannot be responsible for making Venus the awful place it is today."

That’s part of it but the dense CO2 atmosphere is the major issue. Don’t worry, we are trying to get there as fast as we can to Hell as well.

What’s the worst case CO2 on earth? Is it as bad as Venus?

Almost the entire length of the linked article is spent answering that question in detail.

Although there's very little information about the most interesting part. What did they use for their modelling? The results suggest somehow that the modelling includes a lot of manual handwaving...

Don't forget the corrusive atmosphere that is acid and eats up space probes.

Depends on elevation. It’s quite habitable with enough height.

Nitrogen oxygen at one atmosphere is a lifting gas on Venus. You could live in an enormous zorb if you can keep it sealed.

Quite. Its just so.