Because Human Smoke is so much better. Scorpion can be a great character if played well, but H.Smoke is just a much better version of Scorpion, making Scorpion kinda useless.

hoew can i select him in SNES game?

Hold Back+HP+HK+BL+Run after you pick Robot Smoke, until the round begins, and he changes into Human Smoke.

For further information on character ranks, tactics, combos, etc, refer to: UMK3 Character Guides and you might want to check out the boards to get more hands on experience outside of actually playing.

but for some reason there are still people who prefer scorpion instead of human smoke even in competetive games. he is one of my favorite characters but he is nothing compared to hsmoke.

but exactly human smoke is just quicker or what?

I have explained it on many forums, but Human Smoke's walking speed is faster, yes, allowing for easier juggles, however, the more important factor is he has a pop up combo. Starting with a knee lift, Scorpion can do no more than 23%. Starting with a knee lift, Human Smoke can do 50% or more, on every character. That is the difference, and this can be applied in many situations.

For example, Human Smoke vs Scorpion: from a run jab set up, Scorpion can do HK, HK, LK, LK if he releases block, Human Smoke can do, HK, LP, walk in, RH, teleport punch, harpoon, HK, D+LP, D+HP, walk in, aaHP, JK, air throw for 57%, and if you want to get really fancy you can throw a glitch cancel HP in after the teleport punch (breaking the hit limit rule by canceling the move on the frame it connects), and LPLP (sick timing) before the JK air throw making the combo 62%ish vs Scorpion's 23%.

i got it. Thanks Shock

Visual:

http://www.darktemplarz.com/umk3/scorpsmoke.wmv