It is too uniform, which makes it hard.

Everything is a list or an atom. There's no variety and it doesn't take advantage of pre-existing knowledge.

For example, in most curly braced languages a block is very different from an argument list, not so in Lisp.

There's not many visual cues of what function something does, anything can be anything.

Even math expressions are weird (because there aren't any), you cannot leverage years of training reading simple math notation to make sense of stuff.