Its mainly because the TA units attached to the KRRC & RB were based in London. Historically the KRRC & RB recruited from London and the Home Counties even though officially they recruited countrywide. The Rifle Volunteer units outside London tended to get attached to the local county regiment.
e.g. The Leeds Rifles were attached to the West Yorks then RTR
The Liverpool Rifles were attached to the Kings, and so on