Why is there now only one Salmonella species?
Simply put, old habits die hard; physicians and other medical personnel have grown up with the old species designations so will continue to use them.  This is somewhat the reverse of the case with E. coli, where 80-90% of the genome is variable across strains.

Lin-Hui gives a brief history, where strains identified early were given specific names within Salmonella like typhi, typhimurium, panama, etc.  In 1966 it was proposed to lump serovars together among three species S.
choleraesuis, S. typhosa, and S. kauffmannii.  Shortly thereafter in 1972 with the advent of DNA techniques, it was shown they really should be all piled together.  There is one exception, Salmonella choleraesuis subsp. bongori, separated in 1989.