Aurelian instituted the cult of Sol Invictus, and it existed more in tandem to the cult of Jupiter than as a replacement for it. In the tradition of Augustus, most shrewd emperors knew how to introduce new cults without officially replacing older ones. Aurelian, in fact, most often associated himself with Jupiter before finding Sol Invictus.
Diocletian's religious policies have to be understood in light of the politics of the times. The Crisis of the Third Century made ruling circles keenly aware of how fragile the empire really was. It was thought that religious comformity would reunite the empire culturally and return the favor of the gods from a troubled time. This was especially poignant since Caracalla had proclaimed near universal Roman citizenship, making the Roman gods the gods of everyone, in effect.
If certain Greco-Oriental cults were gaining in popularity over more traditional Roman ones, it was because those Eastern gods offered more: a better afterlife, mystical union with a deity, a clear moral code and cosmology, colorful rites and rituals, etc. In the private religious life of the people (as distinct from the state run cults) it seems the common person honored a variety of deities and spirits, new and old.