I agree that was probably one of the reasons. Another reason (and probably more importantly) is aerodynamics. Side-pulls offer a smaller package, more so than any other brake system on road bikes. They're also lighter and simpler to set up. The performance advantage of this is recognized more on a road racing bike than say, a touring or cruiser style bike.

Clearances have definitely gotten tighter though on modern frames and forks starting from the early 80's and on up until now. Aerodynamics was a key factor in all this, so much so that Campy introduced the aero-brakes around 1990, which concealed the brake housings from the lever body and ran them along the bars to the calipers. The aero-brake calipers themselves were also aerodynamically sculped to take advantage of this. (I have a set on my Tommasini). Of course this caliper design only lasted about three years, until campy came out with the dual-pivot brake design.

Sooo,...I think it was aerodynamics, simplicity, lightness, and compact design of side-pulls that prevailed over center-pulls.

so what say ye, my brothers of skinny wheels.