bcrypt, the passphrase-handling part being eksblowfish, takes the user passphrase and turns it into the array of 18 32-bit (4 byte) subkey values blowfish needs for encryption. 18 * 4=72 so that's where 72 comes from. Increasing that limit requires pre-hashing the passphrase down to 72 bytes or less, or chaining multiple bcrypts using each 72-byte chunk of the passphrase. Either of which makes it no longer bcrypt. The problem with pre-hashing is that there is no standard; every implementation uses some arbitrary hashing algorithm. It also adds complexity to implementations.
I would prefer that every hash function, including one that's designated as a password hash, utilize every bit of input to generate the output. If password length is limited, shouldn't it be done earlier, and not silently?