It makes for a better use of resources if the rich are prevented from owning too much land

The problem with excessive land* ownership is that we have an obligation to make sure that our trade does not impoverish other people. When we buy something we have a duty to ensure that we are not buying the last piece of food from a hungry person, and we can do that by making sure that they have sufficient Capital. We can also make sure that the Society within which we live is still wealthy and that resources are properly allocated, and being an excessive landowner prevents that from being the case.

Being a person who owns an excess of property, specifically land, means that, partly because of our actions, we live in a Society which has not properly allocated resources, because of this we have a duty to ensure that we do not own too much land. The cause of inequality (and poor use of resources) is primarily those who own an excess of assets being allowed to do so, allied with their legal ability to acquire an excess of land.

*land in the generic sense, meaning that the concept may be extended to other natural resources that are limited in their abundance.