Answer:
When you purchase a condominium, you purchase a unit, plus ownership of common areas that you own with all the other unit owners. A lender will give you a loan, based on your ownership title to this property.
When you purchase a unit in a co-op, you purchase shares in a corporation and the shares represent your unit. Lenders are less likely to grant you a mortgage for a co-op, since your ownership is represented by shares, not by a title to a unit.