Because that is the rules of this forum, and I need to moderate the posts, which I cannot do if they are not in English.
Ashley has stated many times that all posts must be in English or have an English translation.
If you wish to post in the open forum, then follow the rules. Otherwise, just use email and bypass the forum entirely.
There may also be a native forum for you if you check the following topic:
Vale, entonces voy a traducir mi mensaje para la gente que no importa lo que dije
OK, then i will translate my message for people who don't matter what i said