The unoffical reason is Xamarin wants too much money.
Plus let's not forget Unity was just another 3D engine before they got Mono integrated. Whereas Mono was already being used in multiple areas.
Anyway, this is playing conspiracy theory until someone issues an official statement.
• The C# compiler is dual-licensed under the MIT/X11 license and the GNU General Public License (GPL).
• The tools are released under the terms of the GNU General Public License (GPL).
• The runtime libraries are under the GNU Library GPL 2.0 (LGPL 2.0).
• The class libraries are released under the terms of the MIT X11 license.
and:
When do I need to obtain a license from Xamarin to the Mono Runtime?
We only require licensing for uses of Mono and Moonlight on embedded systems, or systems where you are unable to fulfill the obligations of the GNU LGPL.
There are a lot of Unity customers on our forums using Xamarin Studio already.
In other words, you have to pay a license fee for shipping the Mono runtime in a proprietary game, which is what Unity does.
So Unity has negotiated a license deal with Xamarin for the Mono runtime version they use, and if they want to use a newer version they will have to pay more, which they aren't doing.
Instead Unity has developed il2cpp which means that they no longer have to ship the mono runtime with each game and therefore no longer have to pay the license fee for shipping mono.
Atleast that's what I've gathered.
But reportedly Apple does not allow LGPL in its app store for some reason.
Plus, Unity has forked Mono and they contribute to their own internal version. At the same time they are working hard on their own proprietary il2cpp (to export to C++). The writing is clear: they will drag things as long as possible with using their old version of Mono until they are able to switch to il2cpp.
My personal opinion is that Xamarin guys are better at Mono and that UE4 coupled w. Mono/C# might put quite a bit of pressure on Unity.
Ah, so the deal was before Xamarin bought the rights to Mono from Attachmate/Novell, I didn't know that.
>At the same time they are working hard on their own proprietary il2cpp (to export to C++). The writing is clear: they will drag things as long as possible with using their old version of Mono until they are able to switch to il2cpp.
From what I understood they would still ship Mono as part of Unity (as in development framework), however what they want to avoid is the license costs that comes with shipping the Mono runtime with each Unity game, hence il2cpp, but perhaps I'm mistaken here ?