What makes certain zeolites better at ion exchange?
Zeolites are selective ion exchangers.

What is usually required for a good zeolite is that it selectively absorbs the ion you care about rather than any old ions in the mixture.

That selectivity is a product of the shape of the holes in the 3D structure. Bigger holes will be less selective than smaller holes. If all the ions in the mix fit in the hole the zeolite won't reject the ones you don't want. So, ideally, the holes will be big enough for one ion but too small for another one. This allows selective exchange.

That is about it. It isn't about general equilibration of ions; it is about selective exchange of ions.