[habbababba]

From the links provided so far in this thread one would guess they wanted something that exists in nature and is stable.  Carbon 10 has a half life less than 20 seconds.
And, another link says there was a discussion about using Oxygen 16, but Carbon 12 won out.

From the links we can also deduce that the number 12 is assigned by definition or, in other words, it's arbitrarily chosen (more on that later). So the point here is that carbon-12 is not '12' unless we assign that for it and as a result of applying the previous premise, carbon-10 will turn out to have a mass equal to 10 relative to carbon-12.

From the last link you had posted though, I've come to another conclusion and it may explain why the number 12 was chosen as the standard rather than another number. "Making the amu one-twelfth the mass of a carbon-12 nucleus, however, would lead to only a 42 parts per million change, which seemed within reason." The number 12 did not cause a significant change in the calculations held by chemists and so it appears to me that the number 12 after all does have a specific significance and isn't simply thrown completely at random into the discussion. Am I wrong about this?

[Borek]

12 is a number of nucleons in C-12 nucleus, which definitely was part of the decision. But most of additional arguments behind were simply practical - to make calculations easier.