That is because of other weaknesses of bzip2, not because BWT is worse. BWT generally beats LZ-based algorithms. I would also dispute that LZMA is simpler, at least in compression. Good LZ parsing is not easy.
Check out http://mattmahoney.net/dc/text.html . The top BWT performer is #22. What it comes down to is the way the BWT output is transformed and entropy-coded afterwards, and various other tricks.
bzip2 only does very basic MTF transformation and Huffman coding, which are not very bit efficient.