Speed. 6502s are 8-bit processors running in the neighborhood of 1MHz. Anything you can do to squeeze out a cycle is worth it. Rather than writing something that will repeatedly examine the data, and makes decisions, and then does things, write code that simply does things. Much faster, if you can spare the RAM for the code. Plus if you're really careful and clever, the code to "do things" can itself be the data! Sometimes, anyhow.

Also, the world has changed a lot since then. Interpreters have less penalty on a modern chip than an old stupid chip, because branch prediction, prefetching, and multiple pipelines can really help with them, so it's relatively speaking cheaper to examine data and make decisions and the CPU will spend more time "doing things" as long as the data required and the branches taken are predictable, which they often are in this sort of code. And on the flip side, modern processors really want your code to be static, precisely so that all those optimizations can work well, along with code caches, micro-op caches, etc... constantly changing the code isn't good for performance on modern chips. The 6502 doesn't care how much the code is changing, it just executes the next opcode at the same speed regardless.

Very different world.