The problem with lineups that don't include 2 of the big 3 is that they are replaced with almost purely defensive players who don't require attention from the defense. Kyrie and Lebron go out, Delly and Shump come in, and all of a sudden Love is the only good offensive player on the court. No one can dribble drive and collapse a defense. Teams can heavily shade towards Love in the mid or low post, and will flat out deny him the ball if they can. The other guys on the court can shoot, but only if someone (Lebron/Kyrie) is generating wide open looks for them (Delly, Shump, TT). JR can occasionally ISO or PNR with some success, but that's not his bread and butter. So when that happens you're pretty much just banking on Love seeing single coverage in the post, which shouldn't happen against good defenses. And that's also counting on having a guard with the ability to get him the ball in a good spot, which we haven't really seen.
When Love was the featured scorer in Minny, he had much better options to play off of (compared to the Cavs 2nd unit). Rubio is a brilliant passer who can collapse a defense and abuse spacing despite his inability to shoot. Pekovic is a legit post threat. Kevin Martin had to be respected as a scorer. They weren't great offensive options, but they were options nonetheless. Shump, Delly and TT are essentially non-options that require 90% of their offense to be force fed to them. So is Marion when he comes back, and JR only slightly less so.