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It's likely because lots of new players came in with the new expansion, so the lower level dungeons and trials are being done more? That's what I think anyway.
I know how you feel, but there have been times where I got 61-69 dungeons back to back quite a few times. It of course helps to be in the mid 60s at least as it gives you more dungeons as options :)
Recently on my 62 warrior it's been pretty consistent between keeper of the lake, Sirensong sea and sastasha/totorak.
This is why it's called LEVELING roulette, you only have to be lvl 16 to join. The whole point is to do lower dungeons so new people can still get things done. If you don't like it simply don't do it.
I clicked the title thinking "and by in a nutshell you mean Sastasha?"
There was time I got it for a whole week. ;_;