I think you bring up a lot of good points and it's probably some combination of all of that. I love DP, and they totally deserve the recognition that Zeppelin and Sabbath get for influencing hard rock and metal.

Someone else pointed out a lot of it has to do with the consistency of their albums. Zeppelin released 7 consecutive albums that were absolutely fucking incredible, and their last album is still probably as good as much of DP's work too. Same with Sabbath, their first 6 albums are all complete bangers. Deep Purple took a while to find their stride, their sound evolved a lot, and they had significant lineup changes that altered said sound through different periods.

While they have excellent work that spans several decades, I would say their best run of albums includes In Rock, Fireball, Machine Head and Burn. Note that Who Do We Think We Are came out during this time but I did not include it because I don't think it's as good, save for the song Woman From Tokyo which is a classic. Zeppelin and Sabbath did not have any clunkers during their peak runs. I think that's the difference.