Because I don't have the capacity for maintaining compatibility between too many versions of macOS. For v3, I've stated the goal - not guarantee - of supporting the current and previous versions of macOS.

Since Tahoe is likely to be released in the next month or so, I judged it better to start with Sequoia and keep it supported rather than start with Sonoma and risk needing to drop it shortly after launch.

Because targeting the latest release lets the developer use new APIs/features that are not available on older OSes making development more productive and pleasant.

Of course, there’s a balance to be struck to support users, but macOS update uptake is usually pretty fast.

Because you pay once and that's it.

Now if you did that and used it on your 3 versions out of date macOS, then an update came that only worked on current-2, you'd be pissed.

Mostly only software with subscriptions can afford to keep supporting multiple versions.