Why bcrypt is one-way while Blowfish is reversible?
Because bcrypt uses Blowfish as a component inside an irreversible algorithm.  This is very common—many variable-input length cryptographic algorithms are built from smaller, fixed-input-length components that may or may not be reversible.

One example is the SHA-3 hash function, which is designed following an algorithm known as the sponge construction, which builds a hash function out of a reversible function (a permutation).