Maybe their ages. Guys in Ash' age don't really care about romance. Especially not guys in shounen anime. Their goals about becoming number one and their friendships are 更多 important to them. Even /if/ Ash would be fine with it, how is /Serena/ supposed to know? She doesn't know Ash /that/ well yet.
Another reason would be the fact that Ash is Serena's first crush. Since she hasn't confessed to a guy before, she might not know /how/ to confess and is therefore insecure about what to do, what to say and /when/ to say it.
A third reason would be that she might be waiting for the right moment. She probably doesn't want Clemont nor Bonnie around during the confession. She might want to get to know Ash much better first. The right place and right atmosphere are also important.
She could also be waiting for Ash to hint that he likes her back. Being nervous about getting rejected, Serena might want to be sure that Ash is interested in her as well before confessing.
My last guess is that Serena might not want to "be in the way". Ash is training hard so he can win all gym badges and one 日 become a 神奇宝贝 master. Serena might not want to distract Ash from his training 或者 the battles.
