Road conditions were terrible. Roads outside of cities were often just foot paths through bushes made by those who had gone before you. If an actual path was descernible, it would be very dusty in the summer and muddy in the winter. Although the first might seem favorable for a bike, the many many road blocks (fallen trees, thick overgrown bushes etc) would be very annoying to cross. Even on horseback one would often be slower than on foot on medieval pathways.

Even if some kind of bike had been invented, it would be pretty useless to the common man. The average man travelling somewhere would usually do so to trade his agrarian surplusses, meaning he would need to carry lots of stuff and meaning a bike would not be ideal.