Pretty simple. Only two seasons in his 11 year career where he played all 16 games. Only one season where he averaged more than 15 carries per game. Only five seasons where he averaged more than 10 carries per game. Six seasons in his career he averaged fewer than 4 yards per carry.
He also wasn't a TD scorer on the ground. Barry Sanders only had 99 rushing TDs, while Walter Payton, Jim Brown, Emmitt Smith, and LaDanian Tomlinson were well over 100 for their careers. Bush had 36. Plus he wasn't catching a ton more TDs than any other player either. Sanders and Smith had 10 and 11 receiving TDs respectively, the other players were all comparable or higher than Bush.
I think there are two reasons for your question.
1. He wasn't a workhorse. At 6' and 205lbs he was tall and skinny for a modern NFL running back. Emmitt Smith was the modern workhorse at 5'9" and 225lbs. LaDainian Tomlinson was 5'10" and 215 lbs. Those guys averaged ~19 carries per game for their careers with many seasons well into the 20s of carries per game. Jim Brown, Walter Payton, and Barry Sanders all averaged 20 or more carries per game for their careers.
2. His game was predicated around speed and agility but not power. In college speed and agility allows players to be game breakers. In the NFL during the era he played, with elite athletes at pretty much every position, he was going up against LBs that were almost as fast as him plus DBs that could likely out run him. If he couldn't get the ball out in the open field he struggled to break away from the defenders like he could do in college.
He was a player in the wrong era of the NFL. If he had played in the 60s or 70s he might have been as good as Gale Sayers was pre-injury and been a HOFer. But any back after the 80s needed to have power as a part of their game to be truly elite. What is interesting is compare his career numbers to Sayers and then look at the number of games played.
| Bush | Sayers |
| Rushing TDs | 36 | 39 |
| Rushing Yards | 5490 | 4956 |
| Yards / Carry | 4.3 | 5.0 |
| Carries / Game | 9.6 | 14.6 |
| Receiving TDs | 18 | 39 |
| Receiving Yards | 3598 | 1307 |
| Yards / Catch | 7.5 | 11.7 |
| Games Played | 134 | 68 |
Aside from Bush having a large lead in receiving yards and receiving TDs, they are pretty close and Sayers did it in half the games.
