If there was scope to increase rents, then why were they not increased already?
I don't think rising costs to the landlord always results in rent increases. Rents are constrained to an extent by the average salary of the area.
If you incur more costs, but the average salary of your area doesn't rise, and you increase rent prices, you run the risk of void tenancies.
Because if they did, another landlord would undercut them. But with all the landlords facing rising costs, another landlord cannot afford to undercut. Hence the rents all rise.
This is how supply&demand curves work.
Over the last few years, Seattle has added many various laws that are costly to landlords to comply with. This corresponds with a decline in affordable housing. I'm pretty sure the Seattle Council is well aware of this connection. But as the number of renters far exceeds the number of landlords, they know where the votes are, and they sell the fiction to the renters that the landlords will pay.
Supply and demand don't "work" the same when there is a monopoly of a necessary good.
Consider the scenario where there is no more water in the world. All that is left is in a well in your back yard. You can charge whatever you want for the water, because people need it. The limit is what they can spend, because the demand is essentially infinite.
Now, if you had to purchase a more expensive pump to extract water from your well, would you charge more for your water? No, because you can already charge as much as you want.
I haven't seen the data for other countries, but in New Zealand, you can look at trends of rent vs house price and they are not correlated anywhere near the way rent and income is correlated.
Supply and demand do still exist and affect rent, but when supply is low for housing, landlord cost increases have no effect on rent.