Maintenance and Pilots on the ball.

[Band a Lot]

I prefer the saying why are we still flying piston engines.

Mostly cost, the STC mags are about 10% extra of a overhaul cost around 5K (at end of day).
Also about zero support in maintenance and parts.

The new Connies with extended TBO don't come with STC mags as option and the extra hours are better bang for buck than STC mags.

In Oz as soon as one of these companies goes broke product support is gone - so then is your use of them in OZ. But other reasons also, like mags actually do work fine very often.