Well, you answered your question by yourself. The important difference between boost and the c++11 standard is, that c++11 is a standard. So basically everyone had to agree with the methods and functions related to threads. But as you told us already, "Such methods (try_join and interrupt) sometimes are very useful"
So would it have been reasonable to impose that as standard to every compiler? Maybe, but if you really need it you could as well simply use the boost equivalent until it does maybe some day get into the standard.
@Howard-Hinnant suggested: There were voices on the committee adamantly claiming that cooperative thread cancellation could not be portably implemented in C++. At the time boost::thread did not have interrupt. So Anthony Williams implemented interrupt for boost largely as was currently proposed as a proof of concept that it could be portably implemented. This proof of concept was largely ignored by the committee, mainly because so much time had already been spent on the subject that we were at risk of sinking the entire standard because of this one issue.