The FAQs give a reason why it is not part of the subscription
"Can I unlock a 1939 Register household with my subscription?
Answer:
Can I unlock a 1939 Register household with my subscription?
The 1939 Register is no ordinary record set, and bringing you the most significant family history release of the decade has taken a team of hundreds over a year.
Our accuracy on this record set is over 98.5% of readable records, meaning a lengthy quality assurance process, and we have developed new methods of automatically closing the records of those people who are living and younger than 100 years and one day.
In addition, unlocking a household in the 1939 Register doesn’t just grant you access to a record and image. In this, our richest record set ever, when you unlock a household you will also unlock rich, exclusive content giving you an insight into the world of 1939.
Maps will demonstrate how local areas have changed, never-before-online photos related to the individual’s life will add context to their record, we’ll use the numbers in the Register to show a breakdown of the local area, and local and national newspapers will bring 1939 to life like never before.
It is because of the above, and because the 1939 Register is such a special record set that we’re unable to include the 1939 Register in our users’ subscriptions. We have worked to ensure that the cost of unlocking a household in the Register is the best possible value to our users, and we hope that everyone, both existing users and those new to Findmypast, make new and fascinating discoveries in the 1939 Register, the only surviving record of the entire civil population of England and Wales between 1921 and 1951."
Cheers
Guy