In no particular order:
1. Pro-forward bias in Hart (and Pearson) voting
2. Voter fatigue with giving the same guy the award year after year
3. MVP is different from best player
4. Clarke was one of the best offensive players in the league while also being one of the best defensive players of all times. Quotes from the era mention Clarke and Orr as the two guys who can take over a game on their own.
Personally, I think Phil Esposito's Harts against a prime Orr (on the same team, no less) are weirder.
I don't think Esposito's Hart's are too strange from a voters perspective given the forward bias that emerged following the Norris trophy.
In 68-69, Orr was phenomenal, and easily the best two way player in the league and set a new record for defenseman scoring, beating the previous number held by Pilote by 5 points. However, he was not yet hitting the gross numbers he would be hitting just a year later(Effectively doubling his scoring).
Esposito that year set a new forward scoring record, beating the previous record by around 30 points, and along with Howe and Hull, became the first players ever to hit over 100+ points.
His second Hart over Orr is far more questionable. But still, he scored 68 goals and 145 points. No small feat. And back then, people saw Esposito for what he was. A great player. Unlike today, where detractors like Ushvinder who never saw him play will try to demean his accomplishments by calling him a leach.
Regarding Clarke's Hart's, they were not all that strange. The term most valuable player often was interchangeable between "best player" and "More valuable to his team. Clarke was not as good as Orr, but he played for a weaker team and was still excellent by anyone else's standards, thus....
I think Orr's peak is overrated to a degree. Gordie Howe was better offensively, physically and has the huge edge in durability. The only advantage Orr has is superior defense. Then again Orr isn't doug harvey, rod langway or serge savard defensively.
Clarke deserved the 1973 and 1976 hart trophies, however 1975 was a gift. However clarke in 1975 was still better than espo, dionne, lafluer and mahovlich. Them putting up 3-11 extra points doesnt replace the huge edge clarke had in other areas of the game.
Errrr. Orr was not those 3 defensively true, Mostly because he gambled more, and despite usually getting back on time, a guy who does not gamble at all but has near the same defensive skillset will be more valuable defensively, but Orr was still top 10 defensively all time. The only penalty killer who can be mentioned in the same breath as him is Harvey
His defensive advantage over Howe is substantial given the closer offensive game. Orr's peak value is unmatched by any other player in the history of the game.