It started out as free Early Years education under the Labour Government. Since it was preschool education, it was offered to the preschool years.
At first, it was only available at nurseries and preschools. Then Accredited Childminders were allowed to offer it as they had access to extra training, a qualified teacher and a SENCO through the Network.
It increased from 2.5 to 3 hours per session. Labour were planning to increase it from 5 to 10 sessions when their plans had to be shelved due to the banking crisis. Then they lost the election.
Under Labour, it was free education. Under the Coalition and under the Conservatives, it is free childcare.
That's why it's been on offer for 3 and 4 year olds.
It's my opinion that the promise of 30 free hours of childcare was a last minute promise made in the hope of trumping Labour's promise of 25 hours free Early Years education. There wasn't time to consider the details or explore extending it to younger children.