Steric hindrance is not the reason for the low yield of the esterification of tertiary alcohols. In presence of H2SO4 which provides proton as catalysis, the formation of esters derived from tertiary alcohols and carboxylic acid would be shown below by the reversible mechanism of the hydrolysis of esters.
Because of 3°carbocation would be combinated with alkaline H2O prior to carboxylic acid, so it would form tertiary alcohol again, prior to esters.
And the preparation of esters derived from a tertiary alcohol would be attemped with acyl halides under the condition of alkalinity, e.g. pyridine or (C2H5)3N.