Why are tar archive formats switching to xz compression to replace bzip2 and what about gzip?

More and more tar archives use the xz format based on LZMA2 for compression instead of the traditional bzip2(bz2) compression. In fact *kernel.org* made a late "*Good-bye bzip2*" announcement, 27th Dec. 2013 , indicating kernel sources would from this point on be released in both tar.gz and tar.xz format - and on the main page of the website what's directly offered is in tar.xz. Are there any specific reasons explaining why this is happening and what is the relevance of gzip in this context?