AT-ATs were designed to be impervious to fire from standard laser cannons.

As you put in your question, 1.4 FD P-Tower and DF.9 Turrets were mainly antipersonnel and anti-light vehicle weapons (AT-ST comes to mind). But AT-AT was designed specifically to endure laser and blaster fire, and even weapons like proton torpedoes. In fact , only way to bring it down were non standard tactics like the trick with tow cables and tripping. Later versions were equipped even against that.

As far as we know, there were no standard turrets or towers with weapons sufficiently powerful to deal with AT-ATs. I exclude anti-orbit weapons like V-150 Planet Defender which were unique, cumbersome and to expensive to be used in combat against walkers and other vehicles. And even if those weapons existed, Rebels certainly didn't have access to them. So they used what they had, and relied on stealth rather on firepower, hoping that they main base would remain undetected as long as possible, and also preparing to flee if they were discovered. Turrets and towers around base perimeter served to screen it against light scouting forces and possible pirates or marauders, not against full scale Imperial attack.

  AT-ATs were designed to be impervious to fire from standard laser cannons.

As you put in your question, 1.4 FD P-Tower and DF.9 Turrets were mainly antipersonnel and anti-light vehicle weapons (AT-ST comes to mind). But AT-AT was designed specifically to endure laser and blaster fire, and even weapons like proton torpedoes. In fact , only way to bring it down were non standard tactics like the trick with tow cables and tripping. Later versions were equipped even against that.

As far as we know, there were no standard turrets or towers with weapons sufficiently powerful to deal with AT-ATs. I exclude anti-orbit weapons like V-150 Planet Defender which were unique, cumbersome and too expensive to be used in combat against walkers and other vehicles. And even if those weapons existed, Rebels certainly didn't have access to them. So they used what they had, and relied on stealth rather on firepower, hoping that they main base would remain undetected as long as possible, and also preparing to flee if they were discovered. Turrets and towers around base perimeter served to screen it against light scouting forces and possible pirates or marauders, not against full scale Imperial attack.