Why is Benzylpenicillin better in Bacterial Meningitis than Amoxicillin?
Amoxicillin is just penicillin with a hydroxyl group off the ring and an extra amine group..

  

Their mechanisms of action are the same. Their ability to cross the blood-brain barrier are the same. They can both cause seizures.

Both are used to treat to meningitis. The selection of one over the other would be based on the sensitivity of the infecting organism, and the ability to cross the blood-brain barrier in inflamed meninges.

There are better drugs to treat meningitis.

Penetration of Drugs through the Blood-Cerebrospinal Fluid/Blood-Brain Barrier for Treatment of Central Nervous System Infections