Easy, same reason schools don't use 182s. 152 is about 4.5 gph, 172 about 6.5 gph (training use of course) the 182 and cirrus are in the order of 15 gph. A new Cirrus is also almost twice the cost of a newer model 172.
There's a point where price still matters. People will accept training costs of 15-20% more, not double.
Actually there isn't. Efficient training is far down the list of customer priorities. Convenience is number one. Seconded by professional perception, shinyness of equipment, and perception of safety. If you factor in career training, the 45 hour to finish a PPL disappears entirely since many look at any extra time spent in there as simply extra time-building towards the two hundred for the CPL. In that context, how many hours you did your PPL in become irrelevant. Considering kids are dumping down up to a $100,000 for a full school career package - where often its also perceived that newer better equipment is also going to somehow prepare them more for the world of airlines, the 150/152 becomes even less appealing.There's a demand for training that can be done in the 45 hour minimum, too, but schools aren't falling over themselves to offer that.
It should be noted that most of the folks who may be interested in efficient training, also are the ones who can buy their own plane, so the market for the rental student in this category gets even smaller.
One should note that the economics of training fleets in this regard is specific to Canada. With a bigger training market south of the border, and differing taxation and hindrances in Europe, things still make sense there. For instance there's a large enough market stateside to support both schools who still run old 150/152 fleets, and ones that run stables of Cirruses.
