I've read a lot of stories about it, because I'm a fan of the game and also used to dabble in assembly myself. His motivation isn't as crazy as it's often presented.
He used assembly because he had always programmed in assembly on a variety of hardware. He basically had every typical function documented or memorized from other projects. Just as any programmer can remember the statements in a language, he had blocks of assembly code that he could put together to do the same things. Like functions, right? If it's made right and you know what it does, then you don't even need to look at what's between the brackets.
At the time he wrote RCT, he simply couldn't be bothered to start a new collection of scripts in a different language.