Why is Bombardier selling off all of its airliner series?
It's important to understand that Bombardier Transportation/Aerospace, while ostensibly a company separate from the original Bombardier family (which only retained a minority stake when it was split off from Consumer Products - snowmobiles, watercraft, etc), is still actually controlled by the family due to a bizarre dual class share structure where the family retains its minority stake in the form of class A shares that have many more votes per share (I think it's 9 or 10 votes or something like that) than the single-vote class B common stock.  This gives the family control of the company, and makes it takeover proof (also making it very difficult to raise private capital), even though its equity stake is a small minority one.

But the basic problem is the C series and to a lesser extent the Global 7000, just about broke the company.  So broke they were forced to give the program to Airbus more or less for free.  The CRJ line was neglected due to the commitments for the C with little in the way of proper design updates.  This handed over dominance on the Regional side to Embraer (who's newest Ejet is basically a baby C-series technology wise) and CRJ production is winding down (it is now a Mitubishi product - M's interest wasn't in the Regional Aircraft program, but its support and spares organization, where M was hopelessly lacking).

On the corporate side, the Lear 85 disaster and the Global 7000/7500 development was another money hole, almost as bad as the C.  

So in nutshell, since the traditional government money tap the company has traditionally relied on has largely dried up, the family has decided to dump everything (including rail) and just retain the consumer products operation, the separate family controlled company, upon which the whole business was founded, that was spun off some years ago.