The land property tax is intended to address specific issues with land value due to it's fixed availability and role in the economy. For example it's very hard to substitute unproductive land with something else, because you can't (to any significant degree) create new land, and certainly not with the same locational characteristics as existing land.

These aren't attributes shared by stocks or other financial assets. For example it's not straightforward to charge a rent on stocks, and they're not fungible to different economic uses in the same way land is.