Well you need somewhere to keep the interesting data, and then probably somewhere to tell you what that data is. So yes we chose payload and topic as we started from Mqtt world. We do try to stick to payload being the main property so all nodes know where to look by default. These days lots of properties other than topic are used to describe other aspects of the message, so we are not so tied to topic.
Actually it was the other way round. We had the simple comparisons first and then the more complex ones were added later. Indeed another alternative would be to add an option to contains to treat * as a real wildcard. (Visually like the ignore case option in regex)
