Motherboards have been on the same standard size, created by Intel, the ATX form-factor motherboard.
From there, they have somewhat expanded into the Extended-ATX, or made the motherboard smaller for a more compact design, like the Mini-ATX motherboards.
They occasionally use newer chipsets when chipset companies release them.
But all in all, motherboard manufacturers have been staying on the same form factor for a very, VERY long time.
When will they ever change? Well, for one, it may change with the release of the new Nehalem CPU's from Intel.
The Nehalem will use a completely-new socket on Intel motherboards, and possibly a new standard.
One reason that manufacturers may have stayed on the same form-factor for so long is because everything else has been molded around the form factor of the motherboard.
One extremely apparent component molded around the motherboard size is the computer case itself.
The holes on the motherboard tray are drilled exactly in the same positions as the holes in the motherboards for the screws to keep the motherboard in place while the case is tilted vertically.
Can you see one of the large main reasons manufacturers stay on the same form-factor for so long? Think about it: a lot of the components of the computer are molded around the motherboard.
So if a manufacturer uses a different form-factor, those other components will have a larger chance of being incompatible with the motherboard.
Yes, you've guessed right: if a manufacturer uses a new form-factor, they lose their compatibility with many other components, and lose their share in the market.
Large manufacturers are all about gaining profit through minimal means; staying on the same form-factor for a motherboard is a significant one.
However, it does bore us, the consumer, if they stay on a single thing for a long time.
After all, we are in an age where the attention span of a normal person is very short.