I'm sure the designers of PNG were aware that slower entropy coders that gave better compression ratios existed. But there's a reason they didn't choose them: bzip2 is 5x slower than DEFLATE even on the lowest setting. Scanline filtering, on the other hand, is fast, amenable to SIMD, and even parallelizable if the image is properly encoded. It's also easy: the only thing I don't remember off the top of my head is the order ties are broken in the Paeth filter.
bzip2 is an especially poor choice in 2016 given that it's inferior to LZMA in both speed and quality.
thing is, bzip2 is widespread, unlike lzma not everybody has.
on a technical side, lzma is a better choice. I might add a lzma recommendation, however, in soma cases lzma performs worse than bzip2.
also, decompression speeds matter, not compression speeds.
in the end, use what you prefer. the compression is not mandated by the spec.
the main use for farbfeld is also rather thought to be as a piping format. it could also be used for storage, but that's not the main point.
bzip2 is an especially poor choice in 2016 given that it's inferior to LZMA in both speed and quality.