To remove unnecessary cost of dubious value, & to rely on the common sense of adults living together to protect themselves against fire, rather than the nanny state.
It is illogical to require self-closing firedoors in an ordinary 2-storey house which is rented on a joint tenancy as a shared house (eg to professionals or students) – as is required in HMO-related legislation, as interpreted by some (not all) local authorities.
Such a house is occupied & lived in, in the same way as any (rented or non-rented) family house, with no greater danger of fire. Furthermore, firedoors can be an undesirable feature when the landlord wishes to sell the house to, or return it to (non-HMO) family accommodation. And furthermore again, fire doors are frequently propped in an open position by tenants (esp. kitchen & lounge doors) because they find them inconvenient, & are therefore ineffective. The landlord therefore has the cost (which increases rent) of installing, then removing, a feature of dubious usefulness.
The LACORS fire guidance document does not require them, but not all LAs follow this.
An appropriate smoke alarm system of course is necessary, & I don't disagree with fire doors in a 3-storey house from which escape is more difficult, although there is evidence that sprinkler systems are more effective.