Always a popular God, he was around long before Zoroastrianism kicked off but seemed quite happy to endorse this new religion. Meanwhile he was nicely integrated into the Vedic Hindu pantheon as a VARUNA’s best buddy and partner in worldwide security (see MITRA).
He outlasted most of his contemporaries, and cults devoted to him were still going strong in Roman times (see MITHRAS).
MITHRA has a thousand eyes and a thousand ears. So if you get the feeling you are under surveillance, you could be right.