Because usually when people talk of an accommodation crisis they are referring to a shortage of homes available to be used as a primary residence. It makes sense to consider holiday homes and such as contributing to this problem. Just ask anyone in Majorca for example.

Sometimes people refer to underutilised properties rather than vacant properties for this reason.

I am very interested to know what happens to the proportion of underutilised properties during an accommodation crisis. The most recent linked chart in this thread is very interesting - what caused the steep increase leading up to the GFC (i.e. during a boom - often associated with a 'shortage')?