The main reason there are so many vacant homes is that whereas once investors were buying homes with long term renters in mind, now  many have them on the short term rental market, such as AirBNB. This gives them a much better return and also gives them flexibility as to how the property is used. As a consequence there are fewer homes available for both owner occupiers, and renters. The situation is even more pronoubced in areas that are tourist destinations.