I think a lot of people kind of miss the point about the scripting language... it's not so much that torquescript sucks, or that it's hard to learn. There's a number of much bigger issues:
* Tooling - Sure you can argue syntax hilighting isn't that hard to add to most IDEs, but tooling goes waaaay beyond that. The tooling support for C# for example has code completion, snippets, refactoring, code navigation, advanced debugging, profiling etc.
* Libraries - Say you're writing a game server and you want to store your data in some new-fangled cloud database or other. Or you want to integrate with a hot new game streaming service. There's going to be a C# library for that you can drop in and go. If you're using Torquescript you're probably going to have to write all that from scratch.. probably in c++ (yep, you're going to spend days/weeks in c++ hell worrying about closing connections and releasing resources, even if you only signed up for some high level game scripting). You shouldn't need to burn valuable game dev time implementing/porting stuff that already exists.
* Performance - Torquescript isn't too slow for game scripting, but most of its biggest fans would still normally recommend writing high performance bits in C++. With a fast modern .NET runtime the performance should be good enough that you don't need to leave managed code or mess around going between different programming languages to get the job done.
* Familiarity - Ok, you can learn Torquescript fairly easily. But if does have its quirks... and if you already know C# its one less thing you need to learn coming in to a new engine.
* Productivity - C# is a much more feature-rich language with much neater and more productive ways of doing things. It also has tons of official libraries for working with data, collections, system interactions etc.
* Maintainability - C# makes it very easy to structure and organise your code, and to find the bits you are interested in when needed. There are well documented and tried and tested best practices for code structure that make working with large projects much easier.
* Security - Ok, so .NET isn't the most secure language and can be decompiled, but decent obfuscators exist that do a reasonable job of preventing casual snooping. Torquescript projects pretty much just lay their scripts bare for anyone to fiddle with or copy - and if you want to obfuscate it, you're going to have to write your own obfuscator, because again, its a niche language.
Sure, other engines have their own quirky languages too... but as you also noted, most of those have died out now in favour of mainstream high-level languages. When a lot of older engines were made, being able to embed mainstream languages wasn't really a thing because the open source runtimes didn't exist. Now that they do, people want to use them.