It's a simple case of math when it comes to how these glitches and cheeses are found, there is only so much feedback you can get from a closed beta test, essentially they had a total of 3 days before launch of an open beta, you can find out a hell of a lot more with 1,000,000 people doing things rather then a few hundred or couple thousand.

This one was actually found completely by accident, though. Somebody for real got booted right when Crota went down and he stayed down. Using this knowledge, somebody else came up with a way to replicate it. This is not something a team would most likely find during beta testing.

Good point.