High fuel prices wrt discretionary income (can either be low discretionary income, high fuel prices, or both), or legal restrictions as per SVOboy.
Of course, a massive media campaign, with celebrities being picture in the magazine riding bikes etc. Which would require there to be massive amounts of money being made off bikes, which have minimum maintenance costs, meaning minimum money to be made off people regularly, meaning no advertisersing money... ain't going to happen sorry. Would need a benevolent dictator for that to work.
"Every body perseveres in its state of being at rest or of moving uniformly straight forward, except insofar as it is compelled to change its state by force impressed." - Isaac Newton