Why can't I update my iPhone 6 Plus to13.3?

It's not, sorry. The Dark Mode is a feature of iOS 13 and iOS 13 requires atleast the 6s or 6s Plus (looks similar to a 6 or 6 Plus) or the SE.

See following support articles:

Dark Mode (What's new in iOS 13) https://support.apple.com/guide/iphone/whats-new-in-ios-13-iphfed2c4091/ios

iOS 13 and iPadOS are compatible with these devices https://support.apple.com/en-us/HT210327

Your phone is up to date. The iPhone 6 cannot be updated beyond iOS12.4.4. You need at least a 6S to get iOS13.

The iPhone 6 is not compatible with iOS 13. iOS 12.4.4 is the highest possible.

Is too old this is the reason

The exact reason is that your iPhone 6 Plus doesn't have enough RAM to support iOS 13. Your phone is too old, and you need a newer iPhone generation to get iOS 13.

That feature requires iOS 10..13 that your iPhone 6 can't install.