All the Sherman variant are great, yet many of them see little action simply because the M4A3/M36 combo is too cost efficient.
Because they are great on paper only. Ez8 is a great tank on paper but completely shut down by Panther and Jpz4 for OKW and panther for Ostheer.
If you buy a tank with better AT than AI and a bit more health but that is countered by better tanks few minutes later then you stop building it and prefer a version with better AI than AT because at then end the result will be the same except you'll deal more damage to infantry on the mean time.
This situation has nothing to do with Jackson. Even if you remove the M4A3 people will switch to scott with Jackson.
Saying that, I agree that increasing stock sherman AT and reducing Jackson strength vs mediums is the best move possible but it should be made uppon OKW's Pz4 and not Ostheer's one.