C99 not default C- version for GCC?

Question

Why does not GCC compile the C99 by default? I mean why is it necessary to add --std=c99 flag everytime a code in C99 is written?

Answer 1

Perhaps because it still isn't fully implemented - see C99 status.

It also could be argued C99 features haven't been widely adopted, although that's something of a circular argument.