There is actually a bus service already in the spring, summer, and fall, and it is super cheap ($20 round trip or something). And they've got decent frequency.
They depart from a couple of places, including Crowfoot station. I could technically "walk" to Banff because I live in Tuscany and can walk to the LRT, go one stop, hop on a bus to Banff for the day.
Of course I haven't yet, but I could.
The pandemic has been a factor as well.
"Rail bias" is a real thing though. All other things being equal, people will almost always choose train over bus. And passenger trains are, let's face it, way sexier than a bus. Is that reason enough to spend billions on a train? Of course not, but there are also other factors to consider like the airport link, efficiency, highway traffic, funding sources, etc. It might be easier to get multiple levels of government on board with a train vs. busses galore, even if it costs more.