Why C-forkbombs don't work like bash ones?

That C program is tiny, seriously tiny. In addition, fork()'ing a program like that is very, very efficient. An interpreter, such as Bash, however, is much more expensive in terms of RAM usage, and needs to access the disk all the time.

Try running it for much longer. :)

The real cause for this is that in BASH the process you create is detached from the parent. If the parent process (the one you initially started) is killed, the rest of the processes live on. But in the C implementations you listed the child processes die if the parent is killed, so it's enough to bring down the initial process you started to bring down the whole tree of ever-forking processes.

I have not yet come up with a C forkbomb implementation that detaches child processes so that they're not killed if the parent dies. Links to such implementations would be appreciated.

In your bash forkbomb, you are putting new processes in new background process groups, so you are not going to be able to ^C them.