I don’t think it’s just some random pattern that the second eps in each trilogy have dropped off so much. Sure part of it is because the first have always been phenomenons, but in the case of AotC, by that point there was a significnat backlash in the fanbase against TPM that surely affected Clones. And in ESBs case, the movie was not met with universal praise among the ga at the time of release (or at least according to those who were around to remember it). So now we have TLJ, which also has seemed to split the fanbase, and hence is looking likely for another huge second installment drop off.