This is because increasing numbers of local authorities are using article 4 directions available under the planning system to make landlords have to apply for permission to let their rental properties to groups of 3 or more tenants.
Many local authorities are concerned that so called 'ghettoisation' of HMO properties produces undesirable changes in areas formerly dominated by family housing.
Councils currently imposing restrictions on HMO properties according to the NLA are:
Bournemouth, Brighton, Bristol, Canterbury, Exeter, Leeds, Loughborough, Manchester, Milton Keynes, Newcastle, Northampton, Nottingham, Oxford, Portsmouth, Sheffield, Southampton, Thanet, Warwick, Welwyn Hatfield, York.
It's no surprise that many of these towns are big student centres as this problem is often associated with student living.
Discounted landlord insurance