Because the Pak didn't have it. Their technology was not that advanced, really, at least in Pssthpok's era. (If they built the Ringworld, some must have been more advanced, at one point.)
At the time Pssthpok showed up in Sol System, humans had invented the ramscoop, but didn't have manned ones yet; on the other hand, Brennan considered Pssthpok's ship pretty poor:
It's not that good a design, I could improve it blindfold, but you could buy Ceres with the monopoles!
And figuring out the tree-of-life/thallium issue was a serious difficulty for the united childless protectors of Pak under Pssthpok:
Oh, that's no mystery. Though it had the protectors of Pak going crazy for awhile. No wonder a small colony couldn't solve it.
Given the extreme intelligence of protectors, that implies their knowledge of biochemistry must have been (at the start of the project) below ours now.
The "surviving Suicide Night" thing, if correct, must have been established in much later material & wasn't part of the original idea. (Given that the Pak are very biologically similar to Earth life, they must be descended from Thrintun food yeast - like the Kzin, who can eat humans - and thus be much later than the Slaver era.)
Even if it's now canon (and IIRC the Man-Kzin Wars stuff generally isn't), the Pak tend to lose technology:
I told you, they can't keep their technology. Whatever can't be used immediately, gets lost until someone files it in the Library. Military knowledge never gets filed; the families keep it a deep, dark secret. And the only ones to use the Library are childless protectors. There aren't many of them, and they aren't highly motivated.