The reason for not seeing many 220 Swifts IMO is a shift away from a the type of shooting it was designed for. When the Swift was brought on the scene the primary varmint hunting was groundhogs, crows, and other vermin of that type. Typically it was low volume shooting, maybe a dozen shots on a good day now the emphasis is on PD's and ground squirrels, high volume shooting.
While the 220 Swift is a good cartridge it is out of its element when it comes to high volume shooting, it burns too much powder, has too much recoil, and burns out barrels quickly when firing 300 - 500 rounds per day. It has been replaced mostly by smaller cartridges that burn less powder, with the with higher BC plastic tip bullets that are available the smaller cartridges give up very little in the way of performance to the Swift.
drover
