Rent receivers are generally sent in when a landlord is unable to meet the terms of their contract. It doesn’t mean they can’t pay anything (although often insolvency is behind it), sometimes the landlord is on an ‘interest only’ term that reverts to capital and interest and the uplift in cost means there is no way they can meet the increased commitment.
The issue is also more common in properties with equity because the bank don’t stand to suffer a loss in that position (they do in properties with negative equity), it’s also used as a more coercive approach to borrowers who want to hold on to trackers, as ‘interest only’ is often extended for people willing to forgo that aspect of their contract.
Unbalanced taxation on property is also a concern, the ability to tax a cash flow loss on residential property makes it a difficult trade off of ‘who shall we upset’ the choice being the bank or Revenue (most opt for the former).