Is it? We've two new central defenders and a new left back, we even tried a new goalkeeper with no improvement. I'd say the correlation is that the set up hasn't really changed.
It also doesn't explain how Klopp averaged over a goal a game conceded in Germany as well.
It is probably a combination of things. Few things are ever rarely due to one variable or factor.
The quality of the players in our defensive unit have been questionable. Evaluate the players brought in under each manager who's main skill is in defending of some kind : Mignolet, Sakho, Illori, Toure, Vigoroux, Moreno, Lovren, Clyne, Bogdan, Gomez, Manninger, Klavan, Matip, Karius - hardly a list of top class defenders. Some of these players were never good enough to playing in the league for ANY team let alone a team challenging for top 4. There wouldn't be a single player here that you'd explant into a champion team e.g. the title winning Barca, Madrid, Munich, Man City, Chelsea or Man Utd teams of the past. Furthermore, the ones that have been better than bang average or useless have struggled with injury.
Consistency of selection is another thing. We have been hampered by injuries quite a lot. Whether it be Agger, Sakho, Lovren, Matip, Klavan, Toure, Johnson, Enrique, Flano, Kelly, Gomez, Illori etc. We've struggled to pin down a consistent back 3 or 4 that develops a good understanding. There's also been some chopping and changing going on too, as well as some experimentation. For instance, playing different players on different sides, playing midfielders in defence, playing 3 at the back, playing centra halves at full back at various times.
The personnel around our back 4 have generally been variable too. Mignolet has been far from secure in his post ever since arriving. And I don't think we've actually gone with a out and out DM for many seasons now. This is in stark contrast to various title winning sides who generally are very consistent with their selection of GK, 2 CB's and a proper DM.
We can look at our tactics too. Yes we play an attacking aggressive style but then employ players unsuited to those roles, provide inadequate protection to our centre halves who are not well rounded players anyway, and then have an expectation that we won't ship goals. Clyne is not an attacking fullback. Milner is not a fullback at all and yet has probably been our best LB in recent seasons (which says a lot). The actual attacking fullbacks that we have had either haven't cut the mustard defensively (Johnson) or couldn't even get selected (Enrique, then Moreno). We've never had a proper DM to protect our CB's, and our GK has been someone who prefers to stay on his line and stop shots rather than be proactive and interact with the back 2 or 3. The season that we actually did well in, we generally played a RB at LB (Johnson) or played someone who we hadn't even signed permanently (Cissokho) and none of it mattered because we had the most fearsome attacking force probably in Europe.
If you consider all this, I don't think it is entirely surprising that we've struggled to keep clean sheets and limit goals conceded. What can we actually remedy? Injuries I don't think we can do much about to be honest. Tactically I think the manager has to decide what style they want to play. This could be either an ultra-attacking system with advanced fullbacks and no DM, or maybe have more balanced fullbacks with a specialist DM in front of our centre halves. Whichever they should choose, they should then go out and get players who are actually good in those roles, and are better than the rest. If we go for the former, then make sure that we actually get fullbacks that will contribute goals and assists regularly, and put the players up front that will play to that strength.