The main reason, of course, is that the current standard for third basemen is almost insanely high, as has been documented. If you exclude Pie Traynor and George Kell and Freddie Lindstrom, three Jim Rice-esque poor choices, the current cutoff sits somewhere around Hall inductee Jimmy Collins, right about here:
Jimmy Collins: 58.5 career WARP3 (Baseball Prospectus), 52.9 career WAR (Baseballprojection.com), best 3 WARP3 8.0, 7.2, 6.5 (21.7 total), best 3 WAR 7.4, 7.1, 6.5 (21.0 total).
Career translated stats (for fair comparison): .279, .337, .455, 108 SB/54 CS, about 7-8 Gold Glove-caliber seasons.
Collins probably belongs; after all, the cutoff for 3B has been too low because of the notion that good hitters in the LF/RF/1B can play third (not true) and the notion that it's a less valuable defensive position than short or second (certainly not true about second).
Also, Collins' career WAR total makes him the 148th best position player of all time, and his peak was higher than many individuals on that list. His total has him hanging around with these individuals, none inner-circle types but all worthy of consideration especially when you account for defense (HOF and HOM mean Hall of Fame and Hall of Merit, respectively). Collins was right about as good as Enos Slaughter, Bill Dickey, Joe Gordon, and fellow 3B Stan Hack. Jimmy Collins: not a HoF travesty, pretty valid minimum standard for 3B. A guy with his career value and a decent peak tends to get into the Hall.
Robin Ventura was better.
Career WARP3 66.4, career WAR 55.1 (139th), top 3 WARP3 8.6, 7.8, 7.5 (23.9 total), top 3 WAR 6.7, 6.1, 5.8 (18.6 total).
He was worse than Collins only at their utter peak, and only then by WAR. Ventura too had many Gold Glove-caliber seasons (including two utterly insane years of 20+ defensive wins, by both metrics, in 1998 and 1999). His translated line: .280/.373/.472.
The best thing about allowing Ventura into the Hall would be finally fixing the issues surrounding third basemen. WAR and WARP3 heavily deviate on other 3Bs like Sal Bando, Buddy Bell, and Graig Nettles; both agree that Dick Allen, Darrell Evans, and Ron Santo are getting totally screwed.
Point is, look. There have been a lot of 3B not admitted to the Hall who deserve it. Ventura was one of the top few in the league over the course of his career (for the latter half he trailed the brilliant Scott Rolen, whose career one can only hope voters notice better than Ventura's despite similarly bad counting stats, and the great Chipper Jones, whose value will be easier to appreciate because it came from hitting and not defense). Ventura's also a near-perfect cutoff point for discussions; he bifurcates that group of 6 unadmitted 3B depending on which metrics you use, and it wouldn't be a travesty for all *6* of those individuals to gain entrance to the Hall.
Seven votes, and almost no outcry from the sabermetric folks?
Robin deserved better.