Why do you suppose that this is the case?
If Head Start is effective enough to receive continued funding, why wouldnt we choose to fund it at levels that would guarantee a space to every eligible child?
In the 1990s, Congress passed Healthy Start, a program offering prenatal care, childcare, home visiting, and other services to low-income families for children from birth to age three. But in 2013, its funding was so low it served less than 5% of eligible children.
Why do you think Congress never followed up the authorization with the appropriation of sufficient funding?