I didn't write the rule, but my educated guess is that at the time of writing, Excel BBs were the only available "bio" BBs.
Now, the rules don't say this, but my guess is that the intention was to ensure that bio BBs are used.
Personally, I wouldn't see a reason not to allow non-Excel BBs, so long as they were also bio BBs. That isn't what it says right now, though.