Not sure, but during most of the life of the Airsporter marque people chose .22 as the hunting calibre, because rifles were generally more powerful in that calibre. For example, an ordinary Airsporter would do 10-11 ft/lbs in .22 and about 9-9.5 in .177. People would not buy an Airsporter for bell-target or other target shooting, so none were sold in .177 for that group, it was sold as a man's hunting rifle - in the man's hunting calibre! It might be difficult to comprehend nowadays, but the Airsporter was seen as THE top of the range British sporting air-rifle for a very long time - competing with the less powerful Webley Mk 3 and the superior, but more expensive, Weihrauch HW35.
When .177 started to become popular, around the time of the release of the Airsporter 'S' (which WAS a full-power .177) people started to switch to foreign makes of rifles in droves, and the declining number of Airsporters that still sold were bought by traditionalists who only bought British rifles, and only in .22 calibre. Very few Airsporters were seen in FT, even in the very beginning, and even then most of those were .22.
Therefore there is a 10:1 ratio of .22 to .177 Airsporters. They are rare, in fact I have never seen one! Although they come up very occasionally on the BBS.
Good luck looking for one, and if you have one, well done!