What is the main driver in getting a "professional" program like Photoshop vs paint.net?

I think GIMP would be a better comparison than paint.NET, as it has a huge feature set.
I guess most people choose Photoshop for its ease of use and because of familiarity - it's generally used in arts and design schools, so most students know it, whereas Gimp requires a new learning curve - including learning about its existence in the first place.

Photoshop is very matured - there for ages. I got the subscription. As to the comparison, I can do whatever I can imagine in photoshop. No limits. But paint.net is a basic app with all the basic photo editing functionality - good for beginners and more. Professionals still prefer Photoshop.

I think its more mainly aimed at the user. Different users prefer different software and have their own unique way of doing things, for example I personally love using Jetbrains IDE's instead of something like Eclipse or Visual Studio. It's also more aimed at the line of work someone is doing, some people using Photoshop or something from Adobe tend to be doing more in depth, hand drawn with these pieces software.

Comparing Photoshop to Paint.net there is a massive difference in tool sets that the end user can use. Paint.net is more geared towards those who are looking to do something quickly and not in detail. Whereas Photoshop is developed for those who tend to do artwork and Photo editing as a career or intend to take it to a career base.