One way to think of it is like this - every multi unit building in the area is either a collection of condos or a multifamily single entity. The collection of condos each have their own deeds and are taxed individually. The multifamily only has one deed and one tax base.

Also, the same building sold as condos is much more expensive than the exact same building as multifamily. For example, a multifamily on my street sold for 1.2m and was renovated a bit and is now being sold as condos. One condo is going for 1.4m and another for 1.6m. no one would pay 3m for the whole building, but plenty of people will get into a 1.4m condo.

The coop approach allows people, who would otherwise not be able to purchase the individual condos, go in together to buy the building as a multifamily but as otherwise unrelated people. The overall price of the building is lower than if they were sold as individual condos and the tax base is lower.