Because he's not as good as those two? FWIW, Hakeem also played nearly half his career in the 80's/early 90's, so it stands to reason he'd have more "big games."
Barkley and Malone didn't get many blocks because they were terrible/average defenders. Not to mention, they played the majority of their career in an era where the PF had a very different role than it did in Duncan's era (and Tim Duncan dominating the mid-00s at the PF is primarily what forced the change. Competing teams started to employ more long, 7 footer post oriented PFs to match up with Duncan. And once again we're seeing the PF position redefined with more "stretch 4s" who can shoot from outside).
And lol at using assists, which if you watched any Spurs game in their championship era, you'd understand why Duncan didn't tally a lot of big assist games (although he did lead the Spurs in dimes during their '03 run). And no, assists do not quantify passing ability. Larry Bird was tens times the playmaker John Stockton was, but averaged much less assists. If you're a ball dominant stat- , you can easily rack up the assists in the NBA.
And I don't care who features number one in your arbitrary "big game" definition, but for arguments sake and to show you how re ed your reasoning is:
http://bkref.com/tiny/Iajj4
Duncan is tied for number one all-time in playoff 20-15-5 assist 50% shooting games.
And tied with your hero if we bump up scoring to 25.
http://bkref.com/tiny/dmXXe
And when we exchange shooting percentage for 1 block (I filtered for only 1 block so it wouldn't

Be unfair to Barkley

), Duncan ranks alone at the top.
http://bkref.com/tiny/zshWb
I await your spin.