About websocket

Hi all:

I know thingworx use websocket for long-term connection and bi-communication for message ,

I also learn that websocket is base on socket  and is a application layer protocol of higher level

But I'm wonder why not use nio long-term connection of lower layer(based on tcp) to do this I think this has a better performance.

I'm not sure is this is right or wrong, can anybody help to figure out this problem.

Thanks in advance.