Why then would the industry try to push one wheel size?
Answer: Profit -

Push, Push, Push
In recent years, manufacturers have been pushing 700c wheels even on their smaller bikes. This has led some to assume that 700c wheels may be faster or better because they are bigger. Some shops even tell their petite customers "650c wheels are slower" to scare them into purchasing a bicycle that they stock (nothing scares a cyclist like the word 'slow'). As a result, I find myself explaining 700c vs. 650c wheels several times per month. After just completing a series of emails with a 5' tall woman who was unhappy with a 700c bike that she had purchased (not from us), I decided that it was time for a series of articles on the subject.

There is a good reason to push for same size wheels on every bicycle, but it's not performance or speed.

The reason the industry wants to push 700c is that it's easier (ie. more profitable) to spec bikes if all your bikes use the same size of wheel. When you slip a 650c in the mix for your smaller bikes, for instance, now instead of 1,000 of the same 700c forks, you're ordering maybe 800, and then 200 of the 650c forks. This same thing holds true for rims, tubes, spokes, carbon wheels, rim strips and tires. For high tech racing bikes, wheels are carbon and require expensive molds to make. It could cost an extra $30,000 just to make molds for 2 wheel sizes. In effect, you've just raised your cost on the entire run of 1,000 bikes because you've increased the numbers of individual SKU's as they are called in mass production. If a big bike company orders their bikes in 10,000 unit quantities for each model from a Taiwan or Chinese manufacturer, their costs could increase maybe 20%~25% if they started trying to offer the type of flexibility that a small company like us can give you. Building one bike at a time has its advantages.

It would be much easier for us as well to offer just one wheel size, but we like to offer the petite customer the choice of 700c or 650c. Otherwise, we'd be just like everybody else.

By not offering that choice to smaller riders, they put their dealers in the awkward position of having to sell against those of us who do offer that same choice. We don't purchase expensive advertising in magazines, and the big guys do. This gives them some power to influence the opinions of those enthusiasts who subscribe to those magazines including many who work in bike shops.

Now you know the reason for the unified wheel sizes. If you are a petite cyclist, you can see that it's not for your best interest.