For the same reason that ALL languages do including English, French, Spanish etc.... it's just an aspect of the language.
It seems that you are seeing this issue of having more than one sign for a single object/item/expression etc as a "problem" unique to ASL, and something that is an indicator that ASL is less developed, less refined or lacking in some way - however I can honestly tell you this is NOT at all the case. In fact if you study linguistics you'll find that ALL languages have the exact same "issue" of "multiple names" for objects/expressions etc.
ASL for example has 2 signs that both mean CEREAL, BIRTHDAY, EMAIL etc for the exact same reasons that English has two words (spoken&written) that means:
couch/chesterfield
soda/pop
cart/buggy
thong/flip-flop/sandal
These "quirks" are present in ALL language- and occur in part because healthy languages develop in multiple locations simultaneously - so occasionally a groups of native users in one location will create a slang/new word for an object that happens to have a different word/sign somewhere else. This isn't a problem, it's simply a variation.
If you are teaching/learning with a young child (since this seems to be a concern of yours) - when you talk about that object you simply explain that it has two signs, and that both are proper but some people prefer one way and others prefer another. It's honestly not that "big" a deal. In fact it's an OPPORTUNITY to explain how many people use ASL and how just like in the hearing world their are different accents and different slangs for things ... ASL is exactly the same.
If you are playing a game, or memory recall activities involving:
1)pictures or objects
2)names of objects
3)the ASL sign for the object
If a certain "thing" has more than one ASL sign, ask for both. So for CEREAL if the student/child signs back CEREAL (1), then the person leading the activity (teacher, parent, friend etc) simply says something like "right, there are two signs for C+E+R+E+A+L (fingerspelling cereal) do you remember what the other sign is?" . Then the student/child signs the 2nd ASL sign for CEREAL (2) if they know it, if not the leader shows the other sign, then has the student/child repeat back both signs to help "lock it" in their memory.
For a student learning the language - it's only going to be frustrating if they - or their teacher have decided to MAKE is frustrating, if instead they decide that it is an opportunity to learn more about ASL as a growing language then honestly - it CAN be FUN.
FWIW - Although I've been Hoh since birth, I didn't learn the language of ASL (I knew thousands of signs, but not the LANGUAGE until I was almost an adult) and this mindset makes a tremendous difference in not only how well you learn, but also how you understand the language ... being open-minded and not trying to make ASL into a "form of English" makes a huge difference. Honest.
