A deontological system judges actions as good or bad according to whether or not they adhere to a set of rules or laws. The rules themselves are givens; they are axiomatic, not derived from within the system.
A virtue ethics system is concerned with the character of the actor. If somebody intends to do good, then the action is morally good, and vice-versa. The characteristics of a good person are again axiomatic.
Consequentialism judges actions based on their outcomes. Did the action result in a net increase in some metric of goodness (generally taken as human happiness)? Then it was good.
There are also hybrids, like voluntarism, which says that actions are good if they are good under both a deontological and a virtue ethics system: you have to follow the rules AND mean well to do good. And you can nest virtue and deontology within consequentialism by regarding them as guidelines that generally (but not universally) lead to good outcomes; caching the results of your ethical calculus, as it were.
Which is well and good if you only ever contend with a single ethical system. But the nagging question is: how do you choose between competing systems? If I'm presented with two deontologies, one of which has rules that say that X is good and Y is bad, and the other of which says the opposite, how do I decide which one is right?
Obviously, you can just say "oh, this is the one I grew up with, so I'm going to stick with that". Or pick an authority figure to delegate your agency to. But if you honestly want to give each one a fair shake, to say "what if?" and sort through the implications and weigh the systems using some method that isn't just pure subjectivity, to use some kind of consistent framework that you might expect other people to use to come to a similar conclusion -- or to come to a different conclusion, but in such a way that you'd at least be able to understand where your differences are coming from...
It seems like the only way to do that intercomparison is using something that's basically consequentialism. In which case, it seems like you can take a very handy shortcut and just skip straight to the only self-consistent solution. Happily, it's also the one with axioms that are the closest to universality we can get, being rooted in the commonality of human experience, which makes it a lot easier to bridge the gap between people with major differences. Maybe this is why it's so pervasive in the modern world.