Because it provides:

- Common Channel Signaling - no fixed relation to channels, and no lost robbed bits.

- Unambiguity - There is no possible misinterpretation on meaning of signaling.

- Robustness - In case of transmission errors, messages are resent.

- Interoperability - ISDN is the only true international standard.

- Features: Caller ID, caller-id block, DID, forward, all the features are there already.

- Integration: ISDN can also carry data, even if we kind of forgot about that in the age of cheap broadband.

- Ease of diagnostic: No need for costly circuit analyzers: one simple debug command and the problem is immediately clear.

- Expandability: Only on top of basic ISDN you can have QSIG that is the most sophisticated non-proprietary inter-PBX protocol.