Well, for starters, the service must not be great if Apple isn't confident enough to offer an annual plan, if indeed people are scrambling to cancel their old annual plans.
When it comes to actual bookkeeping and financial tracking, it's technically much easier to manage finances and spending on an annual basis. If this was an expense, it's easier to file an expense once annual than it is doing it each month. Balancing your budget for tax season? It's easier to manage an annual expense.
Tools to unsubscribe or modify subscriptions are easy enough for a small team of developers to implement.
This isn't a new or unique problem: this is something that many other companies have successfully implemented and managed.