Because Sanders is one of, if not the most exciting RB ever caught on film. From a "working with what you have" point of view, he's at the top of most people's list.
Better? Everyone has an opinion based on their own criteria. For me, I look for a combination of quantity & quality production. Though Smith has more yards, Sanders was more efficient AND more exciting while playing long enough for me to disregard Smith's advantage in games played and higher totals.
And like you said, it's a team sport. Some of those accolades come with being on better teams. As a RB, you'll get more carries & yards on a winning team. MVPs are almost always on the best teams. Better teams go to SBs, win, and have SB MVPs.
Other reasons Sanders impresses me more:
-The 2nd half of Smith's career looks average next to Sanders'. In fact, Sanders got better as time went on, rushing for 2K in his 9th year and 1,500 in his tenth. Yes, Smith played longer, but Sanders was at the top of his game longer.
-Sanders was named all pro 2 more times than Smith, playing 5 less years.
-Sanders is one of 6 RBs to rush for 2K. And he did it from the I-formation after many attributed his prior success to the run & shoot, spread formations.
If this was a vote for most productive players, I'm sure Smith would be in the mix. Madden covers seem to be more about popularity. Being arguably the more entertaining player, Sanders will always beat Smith in these types of votes.