My Apple Card was cancelled due to lack of use!?

All credit card companies close inactive (sometimes referred to as dormant) accounts. They do this because having a large Credit Limit cost banks money. Federal Reserve Bank regulations require that banks set aside >reserves< to cover potential losses should the account owner default.

You FICO score declined because you credit utilization across all accounts increased because the account was closed.

It’s not considered a best practice to open a lot of accounts with high credit limits if you don’t plan on regularly using a portion of the assigned limit.