I'll answer your question with a question

Why would you fix your first/business class cabin at n rows when, with identical seats and a flexible divider, you can have your cabin the precise size to match demand ? You can use the same aircraft for an off-peak leisure route and a Friday evening, business-heavy route.

Further, as distinct from the US model, there is no 'tradition' or expectation of elite upgrades to fill the fixed first class cabin.

There was a time when the first fews rows would have had greater legroom, or a 'convertible' seat that would adjust to consume the width of 1.5 seats, but those have disappeared in recent memory.