I asked my King's midwife about this, and she explained that a lot of women aren't eligible to use them (you have to be considered low risk), and of the people that are eligible a large number just aren't interested in using them for whatever reason (e.g. I believe you can't use them if you want an epidural, and many women would rather have the epidural). Of the remaining women that are eligible and want the rooms, a certain proportion will have last-minute complications that mean they either change their minds on the day or end up with a planned/emergency c-section instead. Therefore, if you want a birthing suite and are eligible for one and nothing unexpected happens in your pregnancy there will most likely be a suite available for you.