Two reasons, in order of importance:
1. Hart has been around longer, and was always viewed as the top prize and most prestigious. This doesn't simply change with the introduction of a similar trophy.
Conn Smythe is the MVP of playoffs, and super important/prestigious trophy. Do you think if all of a sudden NHL introduced a second playoff trophy in 2021 but voted by the players, it would top the prestige of the Smythe? It wouldn't, since the Smythe has all the historical prestige and value. Perception is pretty significant.
2. The Ted Lindsay/Pearson trophy has had some absolutely idiotic winners over the years. Way worst than hart (even though Hart winners aren't always 100% right, they're usually close).
I'm all for a good Lemieux ~ Gretzky debate. Hell I might even argue Lemieux > Gretzky at times if the fancy strikes. But in 1986? In 1986 Gretzky had more assists than Lemieux had points. Gretzky had 215 points to Lemieux's 141 points. Who wins the Pearson? Lemieux...
Speaking of Lemieux. You win one, you lose one i guess.....because in 1989, he had 199 points. He got robbed of the hart by Gretzky - which was dumb, it should have 100% been his. But at least Gretzky had somewhat of a decent narrative for hart. New team, helped them shoot up in standings, and obviously a great year too...but the Pearson trophy should have been Lemieux's obviously. Instead of that no, neither him nor Gretzky won it, but Yzerman did. Lemieux 199 points, Gretzky 168 points, Yzerman 155 points...yet Yzerman is voted best player. Dumb.
I think examples like this caused it to lose credibility.