The Wraith never had firewalls to prevent people from beaming in until they encountered it. The Asurans were unfamiliar with the technology because they never encountered it.
We're all expecting some sort of massive drone blitz by the Asurans because of what we saw in Lost City and No Man's Land. Even though both episodes showed that far too many drones were used (wasted). Such a drone blitz would have depleted their power and let the darts take em out and would have been an inefficient way of utilizing drones. Particularly if the Asurans are expecting reinforcements.
They were almost pretty much disposed of the moment Mckay beamed down to the planet. Even if they did detect them, they would not have the chance to react as they were otherwise occupied. Not to mention
Not to mention, they had to tie up this storyline for what purpose we don't know yet. They had to nerf the fleet a little bit in order for the episode to be a success. You see the same thing in SW Return of the Jedi, which parallels this episode A LOT. From the moment the Alliance fleet got together to the getting the hell away from the death......Asuran Homeworld.

Hell, they could have taken out Luke Skywalker, Han Solo, Princess Leiah how many times during that movie but they had to move the plot along so they didn't kill them?