[tga]

How about because it directly works on all platforms, mobile + desktop?

Also it looks like it's the platform the author was most proficient in, which is always a good reason when choosing your stack.

[Angostura]

An RTF document would be more portable, surely?

[tga]

Definitely, only with less functionality and not as pretty.

[opencl]

Works on all platforms except Windows Phone/Windows 10S apparently, because it's completely broken on Edge.