The credit card transaction involves fungible currency, the Bitcoin transaction doesn't do that. As a result the batch operation succeeds for the credit card.

Six equivalent cards, each performing six offline transactions for $5 each results in thirty six $5 transactions, $180 from one account to various merchants. Settlement transactions can complete more or less instantly or take several days, you don't care and your card isn't involved.

What’s the issue with Bitcoin in this context?

The credit card transaction isn't about the specific money. A Bitcoin transaction about this Bitcoin A is different from one about this other Bitcoin B. If I do two offline transactions paying with Bitcoin A, but keep Bitcoin B each time, then even though I received goods worth two Bitcoins, the blockchain says only one merchant gets Bitcoin A and neither gets Bitcoin B. But if I do two $100 credit card transactions, those aren't for the "same" $100, when settlement is completed two $100 transactions adds up to $200.

Yes, but a reasonable bitcoin wallet wont let you spend the same coin twice, even if offline. As far as I know, none of the common wallet software that support offline transactions will let you do this by accident.

Credit cards suffer from a similar problem, the transactions will not necessarily go through when you get online.