A land tax is different from a property tax. A land tax taxes only the unimproved land. A property tax taxes the improvements as well.

With a land tax two adjacent properties of equal size would be taxed the same, even if one is an eight story apartment building and the other is a parking lot. This strongly incentivizes improving the land to its highest and best use. Under a traditional property tax system erecting that building results in a large increase in property tax (as the value of the structure is now taxed) which dampens the motivation to build. Under California's prop 13, the parking lot probably has a tax valuation from decades ago and thus is taxed even less, further reducing the benefits to redeveloping the property.