Evertith has the explaination exactly right. And it is not simple to port ever application to the ARM (RT) platform. If it was Microsoft would have included Outlook in their Office applications for the RT. they did not.
Well don't be a dick. As a matter of fact, RT in the context of Surface does not stand for runtime. It more than likely stands for nothing. Just catchy. See linked Wikipedia article. Note the third line of text.