When the BART/Muni subway was originally built the plan was to extend the subway for several blocks under West Portal Avenue. I think that that plan would have eliminated the L line and just kept the K and M. Perhaps after seeing the difficulty subway construction made for Market Street businesses, the West Portal merchants (and perhaps others) were successful in eliminating the subway, thus keeping the streetcars on the street. In retrospect, the merchants and K and M passengers probably would have been better served if the subway had been extended.