Why Zac's old ult fit the champions kit better
Zac revolves around his passive in team fights. It's fun and interactive to move around trying to pick up blobs in the middle of a fight.
Changing his ultimate from an ability that's helps him move around in fights to one that literally locks him in place while he channels is counterintuitive to the champions kit. Imagine if Fioras ult was a channel so she couldn't move around to hit vitals, or if Draven ult was a channel so he couldn't walk around to catch axes.
New Zac's ilt is definitely interesting and matches the champions theme, but it doesn't match his passive, which is one of the most interesting parts of his kit.