Snape answers this, and many other questions regarding his DE activities, at the beginning of HBP, when Bellatrix accuses him of disloyalty.
"Do you really think that the Dark Lord has not asked me each and every
one of those questions? And do you really think that, had I not been
able to give satisfactory answers, I would be sitting here talking to
you?”
(snip)
“I think you next wanted to know,” he pressed on, a little more
loudly, for Bellatrix showed every sign of interrupting, “why I stood
between the Dark Lord and the Philospher's Stone. That is easily
answered. He did not know whether he could trust me. He thought, like
you, that I had turned from faithful Death Eater to Dumbledore’s
stooge. He was in a pitiable condition, very weak, sharing the body of
a mediocre wizard. He did not dare reveal himself to a former ally if
that ally might turn him over to Dumbledore or the Ministry. I deeply
regret that he did not trust me. He would have returned to power three
years sooner. As it was, I saw only greedy and unworthy Quirrell
attempting to steal the stone and, I admit, I did all I could to
thwart him.”