They have selected certain ages to receive it, presumably because there are too many to give it to everyone at once. The NHS website says:-
"You are eligible for the shingles vaccine if you were aged 70 or 78 years old on 1 September 2016.
From September 1 2016, the shingles vaccine will be routinely available to people aged 70 and, as a catch up, to those aged 78.
In addition, anyone who was eligible for immunisation in the previous three years of the programme but missed out on their shingles vaccination remains eligible until their 80th birthday. This includes:
people aged 71, 72 and 73 on September 1 2016
people aged 79 on September 1 2016
The shingles vaccine is not available on the NHS to anyone aged 80 and over because it seems to be less effective in this age group. "