probably because 700 mm is the standard size, and 650 mm tires are harder to get.

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Funny, I just went to both the British and USA Tri rules and neither say anything regarding wheel size. Where have you heard that there are restrictions?

USAT Link

British Tri Link

I didn't mean it was a restriction. Someone at a bike shop told me that type of bike is no longer made with the 650 cm wheel.

Then are you asking why Tri bikes use different size wheels?  If so then allow me to provide the following quote from a Tri site for a simple explanation:

“Triathlon bikes are designed with a different geometry than road bikes. Triathlon bikes are more forward in their seat tubes (between 75-78 degrees) and are set up lower in the front end to provide a time trial position. A steeper seat tube angle emphasizes the quads more to save the use of your run muscles so that you have more get up and go when you start the run portion of a triathlon. Triathlon bikes come in 650c or 700c wheel sizes. 650c wheels accelerate better and are lighter, having less surface area exposed to the wind than 700c wheels. They are more proportional to riders under 5'10''. 700c wheels have long been the standard in the bike industry. Although slightly larger, 700c wheels offer more comfort and less rolling resistance than 650c wheels and are more proportional to riders over 5'10''. Regular road bikes have a more slack seat tube angle, from 72-74 degrees, and are set up for all-purpose riding. They are designed to corner, climb, and sprint well. The road bike position is more upright and less aerodynamic than a time trial position. A road bike position uses all of the leg muscles to provide as much power to the bike as possible.”

There are still plenty of Tri specific bikes being made with the smaller wheels.  The guy at the bike shop didn't know what they were talking about.
 

ctmt,
Thank you for all the info. It was very insightful. However, this was a quote from a "pro" bike shop, and he told me that, as far as he knows, all tri-bikes are now being made with BOTH wheels 700 cm. I am not a triathlete, but was wondering about this issue because I have a tri-bike that I ride once in while.
I also asked him about time-trial bikes, such as ridden in the Tour. He said it is very common for the front wheel to be 650 cm, and the rear 700 cm.
Next time an Ironman event comes up, I am going to investigate this further.

ctmt,
I did not have any success researching this issue on the Web, but I did call another "pro" shop, and, as you said, the 650 cm wheels are still being made - but in much fewer quantities.
Thank you again for the info.

Commonly Triathletes are looked down upon by many “Roadies”.  Many bicycle shops do not carry Tri or Time Trial bikes and even fewer shops know enough about them to truly specialize in them.  As I previously stated, many manufactures of high end frames utilize 650 wheels in their 05 lines including; Cervelo, Agis, Kestrel, Lightspeed, and Quintana Roo.  I suggest visiting a Tri specific shop or Tri online stores such as Nytro.com or Triathletezombies.com.  Either of those sites should dismiss your Pro saying all Tri bikes now only have 700 wheels.  I’m willing to bet he is Pro Roadie not a Pro Triathlete.

ctmt,
Roadies have a high degree of snobbiness, yes. Especially sprinting champions from Italy. I used to subscribe to the now-defunct Road Bike (?) magazine, and it was amusing to read about all their prejudices and taboos, such as one should never fit a carrying pouch under the saddle, etc.
As I said, I found a bike shop more to my liking, near to my home. I can't swim or run, but would like to get back on the tri-bike to do some short trips out on a nice clean road.
My road bike (really a "Crit" bike") is fitted with heavy tires and inners, and is now used purely for commuting to work on city streets littered with broken glass and gravel.
  

I would venture to say your so called "pro" shop mechanic doesn't know to much about Road Bikes or Tri Bikes.  Both are still being built with 650c and 700c wheels.  However, as stated before the 650c wheels are used more in tri.  You will most likely find the 650c wheels on smaller road bikes where the rider is short.