[Avodah] Reasons for Shechitah

[cantorwolberg at cox.net]

R' Micha wrote:
> Besides, if shechitah were really in order to cause little or no pain, to 
> the exclusion of other meanings in the mitzvah, there would be permission 
> to kill in other painless ways. 

So then why isn't there permission to kill in other painless ways? Or
I should ask, is there any other reasons given other than it is a mitzva.