Aeroplanes like Seminoles / Travelairs were probably also more readily available and the good old Lyc 0-360 will take a lot more abuse.
Whilst the Travelair in Aus is a rare aircraft the Seminole is being delivered new to this country a few every year for the last few years.
The last few PA44 I've flown were under 3 years old with less than 200 hours total time. They also come with unfeathering accumulators which are quite useful in practicing full feather and restarts.
As far as the O-360 being a good engine well thats another argument, they are good at making camshaft shavings.