TBH I sorta doubt that the Imperials actually "tracked" the droids to Mos Eisley. More likely, they put down a small garrison at each of the planet's major settlements, or perhaps concentrated on Mos Eisley due to its proximity to the Jawas and Luke's home. Considering how barren Tatooine is, there probably aren't many such settlements, and Star Destroyers can carry a lot of soldiers.
It seems (from memory, maybe adulterated by the transition from the original version into later 'scene enhanced' versions) that having initially had gunners decide not to destroy an escape-capsule with no life on board¹, they then are bothered enough to investigate where it landed in the global desert.
They pick up signs of droids and think it worthwhile enough to track those to where they get picked up by the Jawas, and track
them to the rendezvous with Lars, in both cases (and, in at least the latter, without any significant reistance being possible so going in all needlessly gung-ho) they obliterate their used-up leads after
maybe having gathered intelligence.
Technically, the trail from the homestead is cold (assuming they can't track the onward progress of R2, nor the sandspeeder that Luke goes out in in his own tracking mission) and whether they encounter the Tusken Raiders or find Ben's sanctuary is unknown, but the sandtroopers cannot be attacking
every isolated settlement or technonomadic crawler on the planet.
The troopers present at Mos Eisley are probably not as authorised to use Extreme Prejudice on initially tenuous links, maybe mirrored by similar detachments sent (or already present as a garrison force, before the space-ambush and co-opted into the search once the fleet arrives and needs knowledgable feet-on-ground) to the various other Moses on the planet, though the logic by which Ben goes to ME to get off-world would perhaps also figure in the military thinking to
specifically patrol ME looking to identify the next node in the droids' onward journey (and/or that of anyone the baton would be passed on to).
Some illogicality there, though clearly signposted by the needs of the plot if you give it just enough slack.
One wonders what would have happened (given the doomed status of the blockade runner and her crew, diplomatic mission or otherwise) had a version of the plans or a hint of their pre-deletion existence been left in the ship (somehow avoiding the automagical knowledge of whether the data had been copied, as it ought to have been, perhaps by copying from a prior system, re-copying to the droid, then destroying the journalised original system to obliterate signs of the second copy of data) to make everyone think that the data had been mysteriously caught prior to further propogation. The
big answer to that is that if it were to have been found then Alderan would havd been implicated for sure and possibly been retaliated against by force. (Good job that didn't happen then, eh?) Maybe we can allow for Leia and anyone else in on the scheme wishing to maintain that sliver of Plausible Deniability, then. At least it left
her alive long enough to be rescued.
¹ Particularly strange, they weren't holding back from obliterating lifepods
with life in them, forgoing the possibility of tracking to landing then capturing someone they could extort useful information from, yet they also forgo the possibility of shooting a pod no less likely to have data stored aboard (at the very least in floppy disk/USB drive/isolinear chip/whatever the SW universe uses, when asking a droid to remember the information and act as a sneakernet conduit is just
too awkward) when it could at the very least have been opportunity for impromptu gunnery training without expending any significant amount of military materielle in taking the shot. They should maybe have tried the whole "shielded from sensors" thing, as per TLJ or some of the tricks OT Han had may have installed in the Falcon. Also, why not just tractor-beam every capsule back into the hanger?