I'm not an expert on plants (I'm an engineer)
but what I can tell you is that
CO2 is the most common and abundant
carbon based gas in our atmosphere.
If we lived on
a different planet, where that were not the case,
then plants may have evolved to use some different
type of gas. But as it stands, here on Earth,
conditions were right for plants to use
CO2 in their growth. Now how exactly
CO2 is used by plants is a question
that I myself would like to ask a biologist, but
the fact that plants do use CO2 is
because of the environment in which they evolved.