Despite both being islands, Isle of Man and Jersey are not directly comparable with respect to their handling of C-19. In early August 2020, Jersey did open up for tourists (subject to them taking a covid test); Isle of Man remained resolutely closed to all but essential travel throughout. They've been cut off for months now. Jersey has demonstrated a flexible approach to opening up when a window of opportunity presents; Isle of Man's preferred route is one of open-ended isolation (barring the special case corridor with Guernsey). I am not criticising either, simply pointing out that the destination which has shown willingness to open up is a better medium term bet for airlines than the one which has not. The last thing airlines need is to take lots of bookings to a destination which shows a marked aversion to accepting any visitors until the whole crisis has passed.
Re the LGW/LTN situation, I would suggest that EasyJet prioritises protecting its LGW slot portfolio more highly than those just about anywhere else. So if they need to ship in more aircraft from other nearby airports to do that, they will. But whilst IOM is a 'no go' for visitors it is an obvious one to chop until policy there changes. For similar reasons, it makes no sense to put the planned MAN-IOM route on sale for S21 when there remains little confidence that passengers will be allowed to travel.
BTW, my comments are not intended to be EasyJet-specific. For example, upto four carriers have been mentioned in relation to a potential new MAN-IOM schedule since the demise of FlyBe. But there is currently just a once daily Loganair E145 service offering connectivity for essential travel. I wouldn't expect any of these candidates to commit further until they can be confident that their prospective customers will be permitted to travel.