It occurs to me that Primera and Icelandair are largely chasing the same market - economy / budget travellers to the USA and Canada. While Icelandair offer connections to numerous airports across North America, the main draws are probably New York and Toronto, and it may be that they have seen a downturn in forward bookings, such that they have decided to pull the plug ahead of the Summer season. After all, Primera are offering daily service to New York, with more seats per day than UA were, and a 4 x weekly Toronto, so again more seats than Transat were.
It wouldn't in the least surprise me if incentives weren't coming to an end as well.
It's extraordinary how cyclical the industry is, often airports, including the likes of Manchester, take two steps forward and on step back - BHX is taking it's step back in 2018 by the look of things.