Because when a significant proportion of the housing market gets turned into business spaces (which is what AirBnBs are), it reduces supply in the housing market, pushing house prices up.

This benefits politicians in several ways:

• Most national politicians are at the wealth level were they have enough savings, excess income or simply receive “housing subsidies” as part of their work (for example, parliamentarieans when they’re from a city other than where the parliament is located) that they are also “realestate investors” alongside their day-job as politiciasn, in which case higher house prices directly make their properties worth more hence make them richer.
• Realestate inflation is generally not counted in the Official Inflation indexes in most of the West. It is however counted in the Real GDP numbers (via a mechanism called Inputted Rent). Give the way Official GDP (which is the Nominal one, i.e. after inflation has been removed) is calculated, housing inflation by feeding into Real GDP but not into the inflation index used to calculate Nominal GDP from it, directly pushes up the Official GDP numbers, so for government politicias housing inflation is a wonderful way to create fake GDP Growth (fake because house prices going up by mere price inflation isn’t really actual wealth - i.e. the “Product” in GDP - being created) which they then parade all over the Press as being the result of their great work in government.
• It makes their very wealthy and very thankful friends even more wealthy and they’ll be very thankful to those politicians. Lot’s of thankfulness available from the Owner class for people who pass measures to make their Assets worth even more, from being welcome to the most luxurious events all the way to millionaire payouts in the Speech Giving Circuit, Non-Executive Board memberships and gold-plated “Consultancy” gigs.