Because even though they all have the same name, they're all independently owned and operated. Which is why one restaurant might offer something that another doesn't carry.
The same reason why if you go a different Walmart you will find different items. If you read the fine print you will read that a item may not be available at all locations . It is up the individual management to decide what the particular store will have. If enough people request a particular item then they will have it in stock.
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