"Why don't you want to play baseball?"
Translation:¿Por qué no quieres jugar al béisbol?
Duo's answer suggested was '¿Por qué no quieres jugar al béisbol?' but isn't this sentence incorrect? I mean is 'al' in front of beisbol necessary? I wrote 'por que no quieres jugar el basebol' and got it wrong.
jugar + a + (el/la) + [sport] is the recommended^ form, and when it is a + el, it becomes al. (^in the official dictionary). jugar + [sport] is becoming more popular, and is permitted. However, it has to be one of these two forms. If you omit the a you must omit the word for the as well.