After doing a bit of background searching on the words "optical" "optical illusion," "visual" and "optics," I have put together some ideas as to why there are two expressions. Please keep in mind that I am not an expert on optics, the history of optics, or Greek and Latin.
I'm going to use the expression "optical illusion" for this discussion.
Optical illusions were known to the ancient Greeks . At that time, anything having to do with seeing and light was lumped into the category "optics." According to the article, there were two ideas about optical illusions, one stating that our sensory organs are to blame, another saying it was the fault of the environment (think mirage.)
The science of optics was mainly written about in Greek. The Arabs took over and made many discoveries, and wrote in Arabic but the main sources of information were written in Greek. Possibly some in Latin, although I am not expert so this is a guess.
The study of optics made its way to Europe, where ideas flourished. However, optics still talked about lenses, the air, light, and the human eye, and people still didn't really know how people see. So if you said you were studying optics, this meant you were interested in how the eye worked, rainbows, lenses, and why sticks in water "bend." Today, if you say you are studying optics, the eye is not included.
The word "optics" comes from Greek. However, the word "illusion" comes from Latin. It is here where I am not certain where to go further because Greek actually has a word for "optical illusion" (ophtalmarati) which seems to mean "eye trickery." The word "Optical Illusion" wouldn't have been used by the Greeks. It is possible that in early writing about this subject many expressions were used and someone who knew both Greek and Latin and had read the ancient manuscripts and current books (written in Latin) decided to take Optics and merge it with Illusion, changing Optics to the adjective with an "al" ending, making "optical illusion."
In light of what we know about optical illusions, a better expression might be "visual illusion." I say this because Optics is the study of the properties and phenomenon of light, and isn't really related to the study of our eyes and our visual system, so somewhere along the line, "visual illusion" became used more. A Google Ngram search shows that around 1900 Visual Illusion started being used and then took off in the 1940s. I am not a historian of Neuroscience, so possibly someone with that background can explain the spike in "visual illusion" at that time. Since then, the words appear to be used the same, with "optical illusion" being more popular.