The other two players besides Mize are Ruth, 1924, and Ted Williams, 1941.
Here are the leaders in the NL in 1939
Interesting that Medwick, a teammate, who hit 14 HRs with a .332 AVG, had more RBIs. The two both split time in the 4th and 5th slots in the lineup. Mize had a .444 OBP while Medwick had .380. So Mize helped Medwick get RBIs more than vice-versa.
Retrosheet does not have complete splits yet for 1939 for Mize (a little less than half of all ABs), but maybe they tell us something. Click here to see Mize's splits for 1939. His AVG-OBP -SLG with no runners on (NONE) were .295-.381-.516. With runners on (ROB) they were .288-.418-.466. His SLG is down quite a bit. Maybe he also got walked often with runners on. With runners in scoring position (RISP) they were .256-.439-.423. If those numbers held up for the whole season, then it might explain the somewhat low RBI total. This is probably just bad luck for Mize.
Click here to McCormick's splits. They are just partial, too. But it looks like they were slightly better with none on.
McCormick had 19 more plate appearances. If Mize had 19 more, he would get about 3 more RBIs using his season's RBI/ PA rate.
McCormick batted 4th all season and the OBP of the three guys in front of him was a collective .372. Using a weighted average (because Mize split time between slots 4-5), the three guys before him had a .342 OBP. Over 2200 PAs (how many the guys before McCormick had), that works out to an extra 66 base runners.
About a third of NL base runners that year were driven in. If Mize gets that rate, that means another 22 RBIs. It probably should be more, since Mize was a much better hitter than average (although his splits call this into question). But adding in the 3 from the lower PA total, gives Mize 25 more RBIs and enough to win the triple crown.
Mize had 92 walks in 669 PAs. McCormick had 40 walks in 688 PAs. So that may have cost Mize some RBIs. Mize edged him 353-312 in TBs and 86-63 in extra-base hits. McCormick had the edge in hits 209-197.
None of the batters in front of either guys stole many bases. The 3 guys in front of McCormick had 111 2Bs, 34 3Bs and 23 HRs. For Mize (the first 3 guys and part of the 4th slot) it was 137 2Bs, 28 3Bs and 32HRs. It does not look like any major differences in terms of guys getting themselves into scoring position for Mize and McCormick, guys with speed (3Bs) or guys who cleared the bases before they got up (HRs).
The Reds runners might have been faster with their better ratio of 3Bs to 2Bs.
So maybe the what cost Mize the triple crown was the higher OBP that guys in front of McCormick had (and if they were getting on more that might have allowed him to get some of those extra 19 PAs as well).