Because the right to withhold one's details unless under arrest is a long-standing and important right that has been undermined in recent years and the current system is a confusing mess for the public.
For a long time, under most circumstances, unless under arrest, a person has not been required to provide their name and address. However now, section 50 of the Police Reform Act makes it an offence not to do so if the officer believes the person is, or has been, acting in an "anti-social manner".
This is clearly open to abuse and has been abused by officers to obtain a person's details, even when the person has not been acting in an "anti-social manner" (whatever that means) but they have, as is their right, refused to provide their details when requested.
Other parts of the Police Reform Act, such as Schedule 4, Part 1 section 2(2), make it an offence not to provide a PCSO with your details, when it would not be an offence to refuse to provide them to a police officer.
The situation is now far too confusing for a member of the public to know when they can and cannot refuse to provide their details. The relevant laws should be amended to restore a person's right to refuse, unless they have been arrested or accepted a Fixed Penalty Notice or agreed to any other similar procedure that allows them to accept guilt and punishment without having to go through the process of being arrested, although perhaps in such circumstances the police should still arrest the person, issue the FPN and then release them without taking them to a police station, which would allow for them to demand the person's details.