Several reasons.

Firstly, I've always considered Yamaha to be a better engine.
Another reason was that the 40hp is exactly the same as the 30hp (weight anyway).
I was also looking fo the best power to weight ratio that I could find.

I didnt consider the service inteval - might not worry about warranty and do the servicing myself anyway.

A final factor was that you can (I believe) flush out the Yamaha by simply putting a hose onto a nipple.
I'm a great believer in that, if something is easy to do you are more likely to do it than if it is difficult.
On the old Avon Jetrib, I installed a fresh water flush but it was difficult to use because you had to lift the whole boat on the passerelle/crane - then run the engine before switching on the flush water - I believe that if you just let it run you risk flodding the crancase. As a result, I didnt flush it as often as I would have liked.

I believe that other engines use muffs and have to be run to flush - might be wrong though.

Make it easy and then you'll do it.