[Nyororin]

It`s simply because English has drawn so much from other languages (both words and patterns) that it has few definite rules. There are always exceptions that make the "rules" hard to remember for a learner... Especially as the exceptions are usually quite common and encountered just as much as the patterns that do follow the rules.
Not to mention the pronunciation discrepancies that come about from melding so many different sources.

Japanese, on the other hand, is largely independent. It doesn`t draw from every which direction and therefore the rules are generally much more definite.