Hi All,
I read a article compare jcs vs ehcache(http://commons.apache.org/jcs/JCSvsEHCache.html), in article, JCS proved to be nearly twice as fast as EHCache in multiple trials for both puts and gets
why liferay don't use jcs ? anybody know it?

Because Liferay leverages other technologies that use caching systems, and the LCD of those is EhCache.