Why are there no gram-positive archaea species, but there are gram-positive bacteria species?

Because of the absence of peptidoglycan in their cell wall.

Explanation:

Archaea and bacteria possess a cell wall that has a protective function. The terms Gram-positive and Gram-negative are used to classify bacteria. This classification depends on the chemical and physical properties of their cell wall and it is analyzed through the use of dye (stains).

For instance, Gram-positive bacteria have a cell wall that is composed of peptidoglycan, which retains the crystal violet dye. This is why, in the microscope or images, we can observe purple-looking bacteria, which means they are 'Gram-positive'. Gram-negative bacteria do possess peptidoglycan in their cell walls, but at a much lower proportion.

Moreover, the cell walls of Archaea are not composed of peptidoglycan. This is why there are no Gram-positive Archeae species.