it would be incredibly bureaucratic to set up tenancies with rents linked to income, it's bureaucratic enough without adding another layer
the majority of tenants are on HB, there are some who work, there should be incentives for those on higher income to find alternative accommodation.
I lived in a housing association flat in the 1990s, they offered an incentive to move out and buy somewhere, we were both working, so got a grant of £16000 towards buying a place, at the time house prices were low, so it was a worthwhile incentive
currently the only incentive is homebuy which isn't a grant, the tenant gets 25% of the value of a home purchased, this gets repaid when the property is resold. a re-introduction of a grant might encourage those who do have sufficient income to buy on the open market and free up social housing for those who can't afford to buy.