Most religions, when and where they developed, were not only a source of power for the priests, but also a convenient set of predefined rules by which people could live their lives.
The problem with this was that most priests altered/made rules in such a manner that they themselves could be benefitted by them. Also, some rules caused problems in some, if not many, parts of society.
The people would obviously want to rebel. But going against religion was, literally, blasphemy.
Those of the sheeple who were a bit more mentally capable than the rest deduced that the only way they could get away with disobeying rules was if they were obeying the rules. Thus, the problem was to somehow make a set of conflicting rules, eitther of which they could follow as need arose. The trickster was, obviously, the first and best recourse, and I believe that this might have played a significant role in their popularity.
Not to mention that the trickster, being trickster-y, as one might expect of a young child with adult intelligence, literally if we see Krishna's case, added a much loved human element to the previously beyond-thee, impervious Gods. This would, obviously, increase popularity with the masses.