It seems to me that this is because it is impossible to fully use this site from your smartphone. You can write a message now but I am 100% sure that it will be spam. In order to write a quality post you need to read the whole thread. It is easier to do with a large screen on the computer. I may be wrong but this is my opinion.
Agreed with you, but it would be cool if there's an app that allows you to read threads you follow, or replies to your thread. It is possible to just view in on the browser in your phone though, and everything is the same, except it being small. Maybe there isn't much point to create an app when you can already view it, with nothing major that can be improved on.