Two Reasons:
1.) The main thing I cared about was immediate help at the expense of long term financial cost. If there was a contract where a team gave a player a large bonus that gets prorated over 5 years, and then a minimum base salary, then his 1st year cap number doesn't tell the whole story. A player could be under contract for a 1 year 5M salary with the same cap number as one under contract for 5 years 50M, and the commitment for years 2-5 get lost in the shuffle not using the guaranteed number
2.) As far as free resources go, historical cap numbers for players on new teams from where they originally signed (IE Babin's Eagles cap hit in 2011 since he's since gone from Eagles to Jaguars to Jets) are tough to reliably dig up.