There aren't years in particular. The question is why they get no attention when it comes to the Hart trophy. No candidates at all in the past few years.
Years that I can think of:
Robinson 77GP 85pts +120 12pts in 14 playoff games
Lidstrom pretty much any year between 99 to 2008. Doesn't get a mention.
Bourque 84 85 86 87 91 94. I realize Gretzky was there but still. No mention.
Stevens never had the points but he was the key cog to the devils success. No doubt about it. He made the whole team better
List goes on.
Mostly because, as mentioned, top defensemen have tended to play for top teams, or they've been outshone by individual forwards having phenomenal years (see: Mario Lemieux or Gretzky, and why Bourque never won).
Robinson's 85 pt year: well, Lafleur won. Lafleur was utterly dominant that year. I think it'd be tough to claim that Robinson was more instrumental to Montreal's success than Lafleur.
Lidstrom 99 to 2008:
1999: Jagr put up 127 points on a marginal Pittsburgh team, and he managed to push that team into a solid position. Led the league in scoring by a wide margin with little offensive support. The closest scorer that didn't have an elite linemate was Yashin (and he was, deservedly, 2nd in Hart voting).
Lidstrom played on a team with Yzerman, Fedorov, Shanahan, Larionov, Larry Murphy. Tough for him to get any Hart nomination with that group.
2000: Pronger wins. Similar reasons to above; Pronger carried the Blues while Lidstrom was great, but was less important to the Wings' success than Pronger was to the Blues'.
2001: this is the only year Lidstrom has a good case for the Hart, I think. Sakic was a solid candidate; Jagr was probably better than Sakic was, but Lemieux muddied that picture up. Lidstrom didn't get much love this time around, which is peculiar since he really did a lot offensively and defensively this season while his teammates had more pedestrian regular seasons. I suspect Lidstrom's subtle game was overshadowed by his all star surrounding cast and coaching.
2002: Theodore deserved it. Iginla did too. No one else was close.
2003: Sakic was injured and Forsberg carried the Avs. That was a beast year from Forsberg. Lidstrom was great, but didn't carry the star-studded Wings.
2004: Lidstrom had a weird year. St. Louis dominant. Makes sense.
2006: as mentioned in an earlier post, Joe Thornton deserved this easily.
2007: Crosby dominant in sophomore year -- easy MVP win.
2008: Ovechkin killed it; 65 goals with little offensive support to carry the Caps.
...
Bourque 84 85 86 87 91 94.
I mean, this is tough. Bourque deserved it, sure, but this is Gretzky's prime.
1984: Gretzky wins. Guess who's second? Rod Langway. Defensemen aren't as neglected as you think, perhaps.
1985: Gretzky wins. (Bourque and Langway 4th and 5th)
1986: Gretzky, Lemieux, Howe, Coffey was the order the voting went. Two defensemen there.
1987: Gretzky 1st, Bourque 2nd (3x as many votes as Lemieux!)
1991: unfortunately this great Bourque year was again surpassed both by Gretzky but also Hull's 86-goal year.
1994: Fedorov carried the Wings. In fact, he played one of the best all-around dominant games probably in modern history this year.
...
Elite defensemen seem to be elite on a consistent basis. It's far more difficult to sustain an elite level of play as a forward. It seems that forwards will have this noticeably peak remarkable season and then return back to merely being great, well above average first line forwards. 86-goal seasons like Hull's, where he carries his club with those goals are too remarkable not to give the Hart too; particularly since he didn't play for an amazing team. Gretzky and Lemieux were too gifted not to see the wider impact on their respective clubs -- they deserved their Harts.
I don't think defensemen are neglected at all, and this is why I asked you to note particular years: because I suspected that in those years, there was always someone a little bit more remarkable than Lidstrom, Bourque and Robinson.