It was obviously easier to level up as a DPS. It doesn't mean it was "impossible" or "very hard" to do so as healer/tank. Mainly it was very slow for holy paladin and resto druids (maybe shamans too, never played a healing shaman). Typically, healers and tanks traded speed for survivability.
There is also the massive "echo chamber" effect, when people just jump on the bandwagon of what they hear repeated around, and make the opinion their own (that's one of the reason hoaxes exist, and why I'm pretty wary of "everyone says that..."). Things like "there was not enough quest to reach 60 in Vanilla", or "only acceptable spec is the best one" when there is alternative spec which are only about 1 to 3 % behind, or "holy priest is horrible to level". They are false, but are hold as true because they were heard so often.
There were two reasons I leveled up my holy priestess :
1) To have fun with the FSR (and that's why I stopped playing her after Cata made the whole dance irrelevant).
2) To check reality vs what people in my guild were saying.
Fact was, one guy in my guild who was actually playing a holy priest was saying "nah, it's actually pretty easy", and many who were not were repeating "it's a nightmare !!!".
Went to see for myself. It felt pretty slow, but on the other hand I never needed to stop and I nearly never died. In the end, it was slower but not THAT much, and I died much less than with any other class except my huntress (and there was a much higher difference between said huntress and any other character than between the priestess and the fastest non-hunter or warlock character).
And when you claim it's idiotic to level as a healer, it seems you're just showing the typical binary obsession with "n°1 or bust", which is at the root of most of these absurd generalisations I spoke above.