why does this card not just destroy/ban but also cancel my spell trap cards?? i activate my effect, opponent plays cosmos, effect is canceled, card banned
happened for example with "gates of dark world"
Because your opponent used a CC that he obtained using money, thus increasing its activation priority against cards that didn't cost money to be obtained.
You should learn how continuous spell or trap and field spell works. This ruling is not just for CC but all S/T remover as well
They need to be face up to ACTIVE AND RESOLVE THE EFFECT. If they somehow remove your card from the field when you active it, it never resolved which mean no effect in the end. You still pay cost btw
In your case, the gates of dark world never resolve the effect since it was the last to be resolved, and just before it resolved, the card disappear which result nullified the effect.