When disolving a iron pill for titration, why do I use sulfuric acid instead of nitric or hydrochloric acid?
In this case, cerium(IV) sulfate is a strong oxidising agent, hence, using $\ce{HCl}$ would likely be oxidised to form $\ce{Cl}$ of higher oxidation state. Therefore, the titre would be vastly higher than expected.

$\ce{HNO3},$ on the other hand, is an oxidising agent and would oxidise $\ce{Fe}$ to a higher oxidation state than expected (of $\ce{Fe^3+}$). Hence, not as much cerium(IV) sulfate would be needed to reach the endpoint, resulting in a titre lower than expected.

Therefore, $\ce{SO4^2-}$, being a relatively unreactive species, is the best reactant in this case.