Cowboy in some ways is super strong and in other ways cripplingly weak. He's just weird. But I can see why he's considered the most effective. He's probably the most consistent, reliable, and, at the risk of sounding like I'm trying to offend cowboy players, easy.
Easy consistent characters are almost always considered top tier, even if their potential isn't as great as other characters. This is how it works in every game. In shooters, the most consistent weapon is almost always the best for example.
Viewing post in WHY COWBOY IS SO HARD TO BEAT?????
Every "easy" character is considered top tier for a while after a game's release. But they typically drop after everyone learns how to play the game.
Take Ike in smash ultimate for example, he was considered top tier for roughly half a year but now he's barely high tier (still a decent placement). He didn't even receive any nerfs but as ppl learnt the game it was getting easier to deal with him.
Ike is no longer considered top tier because he is outclassed by the stronger of the swordies (Aegis/Cloud/Roy), some of his tools are not considered as valuable as before (Nair out of shield), and his weaknesses are more exploitable to the top tiers (shit disadvantage).
No reason to continue playing a character when another character has the same gameplan but is simply better after all.