Why didn’t Asuras have Deep Space Sensors

[Alterus]

Okay so we know Atlantis has Deep Space Sensors capable of detecting ships in hyperspace heading for the City weeks away, given the Asurans are capable of building most Alteran/Ancient/Lantean technology why did Asuras not possess these sensors, you’d think they’d learnt after being hit by those nukes in First Strike.

[Xaeden]

Atlantis has an ability to track Wraith and Ancient ships. It has never demonstrated a pre-established ability to track other ships. This is not precisely the same thing, but in "Sateda" Mckay uses the long range sensors to try to track the subspace frequency of a Wraith tracking device, but to do so he first needed to modify the sensors to their particular subspace frequency:

McKAY: Don't worry, I limited the power so it won't transmit our location to the Wraith. I just needed enough juice to determine the subspace frequency it broadcasts on ... (he types on his laptop) ... and now I need to re-calibrate the city's long-range sensors to pick up on it.

The Ancients probably got their hands on Wraith ships before, they knew how their own hyperdrives work, and Mckay would've had access to the tracking device they took out of Ronon the first time, so it makes sense that those things could be tracked. Unknown ships may be a problem because figuring out what frequencies to calibrate sensors to look for would be a guessing game.

And we know that hyperspace engines use unique subspace frequencies because of the discussion surrounding the Attero device:

McKAY: OK, look. Um, although they're all based on a fairly similar technological premise, each race has a slightly different type of hyperdrive. Ours are based on the Asgard drive with our own little twist; the Ancients had their own particular system; and the Wraith, again, have their own separate hyperdrive technology.

JACKSON: All right.

McKAY: OK, so at its base level, a hyperdrive allows you to travel great distances by entering and exiting sub-space.

JACKSON: OK, this much I already know, yes.

McKAY (leading him back to the central console so that they can see the device in the next room): OK, so, if this machine is capable of actually functioning safely, it disrupts the very specific sub-space frequencies the Wraith use.

JACKSON: So they wouldn't be able to engage their hyperdrive.

McKAY: Well, that's the genius. They would be able to engage their hyperdrives. It's just that their particular channel of sub-space would be destabilised and their ship would be ripped into a million pieces.

This, however, does not explain why the replicators didn't see the Wraith ships coming in the season 4 mid-season two-parter. That's something that I thought would be a problem at the time, but the replicators didn't even have their shields up, so they were caught unaware. The most logical explanation would be that the Ancients figured out Wraith subspace frequencies only after their "break" from the replicators. That would also explain why the replicators turned to targeting human worlds as a way of ridding the galaxy of the Wraith instead of going directly to the location of each Hive and Cruiser. Sure, they're not overly fond of humans and it's a two-birds with one stone situation, but it's also an inefficient, slow means of getting rid of the Wraith. It would've been better to get rid of them first, if they could, and then take care of the humans.