Are the A220s meant to replace the 717s? The -300s are larger, whereas the -100s are a 1 for 1 swap with the 717s for capacity. The -100s have all been delivered, and there are no more on Airbus's order books.

As for why it takes so long, there are lots of airlines that have ordered the -300s, and delivery slots pre-pandemic were very valuable. Up until a certain point, airlines can swap delivery slots (with the airline wanting the earlier slot paying the other airline for it). After the pandemic, the delivery slots have lost pretty much all of their value, so airlines are just taking delivery with the schedule as-is. Airlines will plan their fleet strategy with the delivery schedule in mind, and Airbus has not been hitting their aircraft delivery targets, so there's no point in getting the aircraft any sooner since they already have the plan based on the schedule. From what I've seen of the schedule, Airbus will manufacture 1-3 planes for an airline, then 1-3 for another airline, with maybe 4-5 airlines in the rotation, sometimes more. The A220s come from both the former Bombardier facility at Montreal Mirabel Airport, and the newly built Airbus facility (next to the other Airbus facility assembling A320 family aircraft) at Mobile International Airport (also known as Mobile Downtown Airport).

(*waves hello from the JetBlue forum where I keep track of their A220 deliveries on a similar thread*)