Why the Lotus Sutra is called The "One Buddha" Vehicle

"In the same volume in the Lotus Sutra, the Buddha says, 'for the sake 
of the Buddha way I have in countless different lands from the beginning 
until now widely preached various sutras, but among them this Sutra is 
foremost'. This passage means that Shakyamuni Buddha has appeared 
in countless lands, taking different names and assuming varying life 
spans." (A Sage and an Unenlightened Man).