He got 15 votes from the 16 member "Golden Era" committee. I read a couple of stories about it but I did not see any statements from the committee or its members on why they picked Santo and no one else. I think Santo definitely deserves it. It might have helped that a former teammate, Billy Williams, was on the committee along with long time Santo supporter Brooks Robinson. I have written some posts on Santo in the past. Here are their links:
What Might Explain Ron Santo's Low Hall Of Fame Voting Percentages?
I used some voting formulas I had come up with and it seemed like Santo just did not have the kinds of stats and accomplishments that the voters (writers) liked.
Did Santo Play In An Era Of Poor Third Basemen?
I showed that he didn't. Sometimes our advanced sabermetric stats might be a little mis-leading since we compare players to the league average at his position during his time period. If Santo had been up against many weak 3B men, that could make his value look greater. But I don't think that is what happened.
Santo Was Valuable Outside Of Wrigley Field
I showed that although he hit much better in Wrigley Field than elsewhere he was still very valuable in road games (his career OPS in home games was .905 while it was .748 on the road-this includes the 117 game season for the White Sox in 1974). This was based on the run environment of his era, which was generally low. So although his road stats don't look like much, they were highly valuable.
Was Ron Santo The Best Player In the National League From 1964-68?
I looked at various sabermetric measures. If we was not the best, he was close.
Ron Santo vs. Brooks Robinson And Hall Of Fame Voting
I showed that Santo's performance compared favorably to Robinson even though Robinson did much better in the voting.
Peak Value And Hall Of Fame Worthiness
I showed that Santo's performance from 1965-67 might have been tied for the 24th best 3-year run in baseball history.