That seems to be a (minor) alteration in the original plan, possible due to the storage requirements for the vaccine.

I think it's mostly because NHS organisations are already well organised for distribution of vaccines to staff. They're large organisations with thousands of employees.

Care homes are all mostly small, with a handful of employees.

EDIT: Wales has just said it's going to be really hard getting the Pfizer vaccine out to care homes: https://www.pulsetoday.co.uk/news/clinical-areas/immunology-...