One of the notions behind property tax is to make efficient use of land -- someone simply squatting on a property and not contributing to the local economy is a less efficient use than someone who's actively engaged. In particular, high-density uses are preferable to low, where land value is high.

That conflicts with the notion of neighbourhood and social stability. Kicking out pensioners (or young families) simply because they cannot afford to pay taxes ... is highly disruptive. For a sufficiently growing market, it need not be a necessary problem, tough much experience with gentrification (or its less genteel earlier forms of land appropriation and expropriation) show.

One possible compromise is to allow taxes to accumulate, but to factor against the value of the property itself. When the property is sold (or transfered in estate), those taxes come due, and are debited agains the value of the property. You don't evict those unable to pay, but the benefits of squatting do come with an (ultimate) cost. That cost is also a bankable asset to the government authority, who can consider this a future receivable.