Or maybe they thought they had succeeded in those few instances but were trying to use those specific successes to infect the others? That would explain the Light Quickening, as it successfully made the recipient less violent, unfortunately it only passed on with a death.

There could also be a whole plot line, about how Duncan is as successful as they've managed so far because he has immortal friends and family, and only kills in defense (with the exception of the Dark Quickening, which he recovered from.) So the Asguard try to take him away and back to his people's origin planet. They think: success, he will be happy and ready to restart his species' society. Duncan, however, is less than amused by being kidnapped by aliens and abandoned on some alien planet across the galaxy from his friends and family.

There he is, surrounded by stasis plods containing thousands (millions?) of what are, essentially, the worst of all immortals and who don't even share a culture with him.