It was too large for regionals, too small for mainline. For example many airline union scoping contracts require planes with more than 70 or 100 pax go to mainline which means it will cost more to operate. Airlines don't want to do that. If it's going mainline, they want more seats (think 737/a320 ranges) which the A220 doesn't have.

There's a somewhat famous post from a Bombardier product manager on the internet detailing the history of the aircrafts evolution that played out like the Bradley Fighting Vehicle in The Pentagon Wars. Despite marketing data and customer feedback saying don't build this they kept pushing ahead and the Quebec tax payer footing some of that bill.