Why did older tri bikes use 650 wheels?

There was the unfounded belief that they were faster.  Lower center of gravity, ability to use larger gearing, belief that aerodynamics were better, etc.

Basically the same reason people are using compression socks, power cranks and other mythical speed boosters.

When you actually design a bike around a somewhat steep aero position (78 degrees plus), in order to make the weight balance on that bike proper (for good handling), you have to tuck that wheel well up against the seat tube. It takes a 650 wheel to do that, almost regardless of frame size (you have to stretch the front end beyond what the rules allow if you go with 700's).

But apparently some people (including much of the bike designers) don't care about that, and all they can think of is "we don't want two wheel sizes" and "with integrated headsets, you can get low enough now with 700's". Problem is, getting low enough was never a problem unless you're riding a pretty small frame size.

Thanks for the responses guys.

DT - I remember that you ride a lot of 650s.  Any reason to avoid them?   

650's give you marginally better aerodynamics, marginally worse rolling resistance (both well below other factors). 

Availability of tires/tubes for 650's means you ought to keep a few on hand, and your buddy with 700's cannot loan you a tube to finish out a ride. 

I've got 650's on my (old) tri bike. 

Hey, you forgot to mention transition buckets

I already have my morning cup of coffee and a toasted bagel ready to watch the slugfest that will likely happen assuming the cool kids wanna come out and play.

Yeah -- you might scoop up that cool race wheel I've been eying on eBay...

Nothing legit as a reason to avoid them, unless you're dead set on using 28mm tires. In fact, you'd be hard pressed to find 25's. But there are lots of 23's (which is what I ride).

I think I smell winter project....

Thanks again guys. 

Marketing was able to sell more wheels... anybody remember the oval chain rings from the 80s???

Heh, yeah. Shimano Biopace - sometimes referred to as the solution to a problem that didn't exist.

[offtopic]

Compression socks are not for speed. They are designed to expedite recovery time.

[/offtopic]

Don't confuse those assymetrical rings with Rotor Q-Rings, which have proven benefits.