i think the premier league's gone through a bit of an identity crisis due to a desperation to emulate barcelona's success/aesthetic and transition into a high possession style, even though a) no manager is getting enough time to create a team like that from scratch (except wenger and, well, lol), b) england doesn't have the coaching infrastructure to properly teach it and c) european football is arguably moving on from that style already.
that said, i don't think there's some big overarching explanation really, different clubs have different problems. compared to, say, a decade ago when english teams were doing fantastic in europe:
- liverpool and spurs are worse because they sold their best players and made a lot of silly buys/hired some mediocre managers.
- arsenal are on roughly the same level. still flattering to deceive, naively idealistic, always one or two pieces away from the finished article, etcetc.
- utd are worse both because it's difficult to transition away from a manager with a 26 year reign and the team fergie left behind was one of his worst, with virtually all of the best players getting past their primes around the same time (rio, vidic, evra, rvp, perhaps rooney now as well.)
- city have had endless nasty draws in the CL, if they've become chokers it's because of how incessantly the media has labeled them as such imo. i also don't rate mancini at all, his teams never had much balance/purpose and were only successful due to individual quality. i assume they'll come good soon.
- chelsea have been in a transitional phase as well, and their current meltdown is entirely mourinho's doing. i think they're a good enough team for at least a quarter final spot if jose finds his marbles.
then there's the fact that real madrid, barca and bayern have increasingly hogged the best players; a huge amount of english clubs' exits from the CL over the past few years have been at the hands of these teams. can probably add PSG to that mix too soon.
i actually think the premier league mid-table is probably stronger than it used to be a decade ago, for what that's worth, and i like to think one of them might capitalise on the above problems one of these seasons - i hoped southampton were gonna pull it off last season for a while.