This issue came up over at The Daily Something's Happy Birthday To Mike Piazza. Bill apparently thinks that Piazza got robbed when the award went to Larry Walker of the Rockies. The only thing I can cay about that is "Don't Mess With Texas!":)
The reason is that Craig Biggio of the Houston Astros was also deserving of the award (although both he and Piazza were clearly better choices than Walker). The Rockies actually finished 3rd, 7 games behind the West champs (the Giants). The Dodgers were 2nd, 2 GB. The Astros won the Central, but only went 84-78. The Dodgers went 88-74. Sabermetrically, it looks like Biggio and Piazza both had clearly better years than Walker.
Here are the leaders in Win Shares that year
Now the leaders in batting plus fielding wins (it is the Pete Palmer linear weights measure, from Retrosheet)
8.2 Biggio HOU
7.7 Piazza LA
6.2 Bonds SF
6.1 Bagwell HOU
Walker had 4.4
I do recall that maybe some writers saw that his home/road splits that year were balanced, so they did not discount for Coors. His AVG-OBP-SLG at home were .384-.460-.709 and on the road they were .346-.443-.733.
Biggio's AVG-OBP-SLG that year (home and road) were .309-.415-.501 playing in the Astrodome. He stole 47 bases with 10 CS and played 2B. Walker was a RFer.
Sean Smith has them with the following wins above replacement (WAR)
Biggio still comes out on top, but not by as much. Others already mentioned had
I don't see where Sean lists the year by year leaders, so I will have to leave it at that. Another thing to look at would be the NL leaders in various stats that year at Baseball Reference. Walker does well in adjusted batting runs and adjusted batting wins. I think these are park adjusted. So fielding wins would have to vault either Biggio or Piazza past Walker.
1. Walker (COL) 6.6
2. Piazza (LAD) 6.5
3. Bagwell (HOU) 6.0
Bonds (SFG) 6.0
5. Gwynn (SDP) 4.8
6. Biggio (HOU) 4.3
7. Lankford (STL) 4.1
8. Mondesi (LAD) 3.2
9. Caminiti (SDP) 3.1
10. Snow (SFG) 3.0
Olerud (NYM) 3.0
Hundley (NYM) 3.0