Dropbear ssh is very useful when you have a full disk encryption on a remote server and want to be able to decrypt it after a reboot.
For initrd you generally prefer static binaries. Not saying that OpenSSHd doesn't build statically, but having less code and dependencies makes it easier to statically compile.
But yes, technically there is no reason to not use OpenSSHd, but in practice having a smaller and more self contained binary helps considering that you would want the bare minimum during initrd.