ZEO is a centralized system, it exposed the FileStorage API and store all data in one place. As long as the load increase, the storage may become overloaded (disks, cpu...) which lead to a technical barrier against scalability.
NEO is a distributed system, all objects are distributed among multiple storage nodes so the load induced by the clients activity is also distributed. Client nodes deals with objects that are available on multiple storage nodes and connect to them on-demand, so each storage handle a subset of operations and a part of the load.