Traffic volume. Have you ever been on that section of the street during rush hour?
Centre Street funnels traffic to/from 16 Ave, and to a lesser extent 20 Ave. That's why we have the reversible lanes all the way from downtown to there. If they're so pointless, why not remove them today and dedicate one lane in each direction to buses? Surely that would be a smooth and inexpensive way to improve transit service if what you're saying is true. Or heck, even just save some operational dollars and shut down the reversible lanes so we have two lanes of mixed car/bus in each direction.