Why Calgary will get regional rail = politics. For the ring communities, they get a benefit, Calgary gets the ego boost of being a 'big city'. Province gets to say they brought transit to X# of people (even if the number of cars taken off the road is much smaller). Even better for stoking Calgary's ego, Edmonton will not get a similar system (they will get the stub LRT line to NAIT funded most likely)
Once regional transit is built up, they may even get the gall to build HSR since feeder networks will exist!
All these fit the pattern of a project that will have massive provincial support. The system will also have a name that emphasizes that it is provincially owned and funded.
There won't be many groups opposing this expenditure. Only taxpayers (which opposes everything) and maybe CFIB would oppose, maybe 'friends of parks' (which would put different sides of the environmental movement to fight eachother which the province loves to do) and while they have a constituency, it isn't enough to stop it.
For the size of expenditure the province wants to make, this makes sense, unless the city of Calgary assembles a P3 that can build the SE line for ~$800 million in provincial dollars, regional rail is the next most ready to build project that is something 'new'.
LRT in Calgary has shown it has the ability to generate ridership unlike buses, which is why in last quarter of '07 there was a big increase in transit ridership, but no increase in bus ridership. We aren't going to abandon that philosophy now (but this discussion if it continues in particular should go back on the Cal-Trans thread).