First.... the IDE is written in C#, not C++
Also, we've "had" it running on Linux, but no where in a complete form; that is it wasn't able to load projects or anything like that. This was also some time ago.
Lastly.... porting to Linux is a massive undertaking, lets not pretend it's not. Even with OSX (which is much higher than 10-15% of our user base) having been done, Linux is just a different beast. Yes the OSX build helps, but each platform the IDE runs on needs significant amount of work to port it.
Now lets even ignore that portion of work. The fundamental issue is maintenance. Every time we update something in the IDE it must then be fully tested - and fixed, on all other platforms. Without a significant user base, it's simply too expensive to maintain a Linux build. We also can't just do one build, throw it out there and never touch it again as games can, IDE's change ALL the time, update ALL the time. The dev time is expensive, the testing increases massively. Not only do you need to test the whole IDE every time you do an update, but you also have to test you can build to every platform, make sure it can talk to all the devices etc.
Without tens of thousands of users, it's simply not viable. We either need massive numbers of users begging for it, or we need a directive from on high saying they'll shoulder the expense for "other" reasons.
Doing a Linux port is not a small or cheap task, it never was.