You can imagine that some of it has to do with the fact that governmental functionaries are not always the best judges of aesthetics.
Furthermore, leaders often come up with beauty-killing mandates, like requiring that coins depict dead heroes or, as in the case of Caesar, the leader himself.
Most people barely glance at the designs on coins, and don’t care about them as long as they can easily differentiate one from another.
Also, the fact that making coins is relatively expensive compared to printing bills or stamps, both in materials and equipment, makes whimsical, ever-changing designs rather unlikely.
In fact, officials in most countries insist that coin designers not make the designs too pretty.
Pretty coins drop out of circulation faster, they’re collected, hoarded, and even turned into ornamentation and jewelry.