How to chose between 9V or 12V (or others) DC power supply
So, first of all, you get a power supply that fits your need. If you, for example, have an amplifier that will give you more output power if supplied with higher supply voltage, well, pick the highest supply voltage.


  I do think there is something to do with efficiency and dissipated power in power supplies and/or regulators but I can't really answer the questions above.


More about availability and cost:

Since these voltages used to be so common, power supplies are really cheap for these, because they are manufactured in millions.

Because the power supplies are so cheap, these voltages became so common.

So, if your device can take in anything between 8 and 10 V, you'd probably pick 9 V, simply because you don't have to look far to find a supply.

12 V might partially be popular because back-back-back in the day, that'd be a typical lead-acid battery voltage, so quite a bit of circuitry was based around that.