For me: because my background is mainly Java. While I understand prototype inheritance, I'm not as familiar with the programming style of Javascript. I like being able to create strict class structures and use those. Prototype structure allows for ad-hoc creation of a class. This makes code analysis and tooling a lot more difficult to do properly.

(I do have to deal with the fun that is Perl on a daily basis, but I'm still more a fan of Java style OOP).

Also, my entire professional career has been working on projects that are worked on by 30+ people with a very large code base. When you have large teams using a language that doesn't enforce strict structure, you tend to miss things that could have easily been found at compile-time with proper tooling. This can be accomplished in languages like Javascript, but it requires strict internal controls. Having the language give you that kind of support out of the box is nice to have.