1) Whatever investigation baseball or the senate or whomever would've wanted to launch wouldn't have been allowed to take effect in time to preserve the single-season record. They would've literally had to arrest McGwire to keep him from surpassing Maris.

2) The media DID uncover McGwire taking andro during that season. The problem was that it wasn't illegal at the time and there wasn't even a testing policy in place to officially implicate him for any wrong doing.