Why is bwa-mem the standard algorithm when using bwa?
bwa mem is newer, faster, and [should be] more accurate, particularly for longer reads.

From the bwa man page (presumably in Heng Li's own words):


  BWA is a software package for mapping low-divergent sequences against a large reference genome, such as the human genome. It consists of three algorithms:  BWA-backtrack, BWA-SW and BWA-MEM. The first algorithm  is designed for Illumina sequence reads up to 100bp, while the rest two for longer sequences ranged from 70bp to 1Mbp. BWA-MEM and BWA-SW share similar  features such as long-read support and split alignment, but BWA-MEM, which is the latest, is generally recommended for high-quality queries as it is faster and  more  accurate. BWA-MEM also has better performance than BWA-backtrack for 70-100bp Illumina reads.