Simon Hibbs came up with 2 interesting other possiblities.  These are:

1. Six good reasons why the Pak didn't build the Ringworld.

2. The Tnuctipun Plot

Six Good Reasons Why The Pak Didn't Build The Ringworld

By Simon Hibbs

1. The idea that pak protectors would suddenly decide that thier non-sentient, esentialy helpless breeder stage is better off wthout protectors to look after them is ludicrous. They'd also know that the breeders would mutate from studying the earth breeder colony. Letting that happen is simply unthinkable to a pak protector.

2. The Protectors would have known about the Kzin, Grogs, Humans, Martians, etc and rather than wipe them out, as the Brennan Monster and Phssthpok wiped out the martians on mars, they brought them to the ring and gave them a nice cozy home to live in, right next door to the breeders, and withing spitting distance (in ring terms) of the command centre for the entire ring. Yeah, right, good move!

3. The theory presupposes that the ringworld engineer protectors found the records of the failed earth colony and follwed it's path in secret. It it was a secret, why did they leave records of the eath expedition in the great library on Pakhome? Suppose some protector decided to go on some fool errand to rescue them with a load of thallium oxide, or suppose a pak family decided to conquer earth for itself? They'd have a planet full of protectors from earth right on the doorstep of the ring!

4. Pak protector technology doesn't use automated controll systems. Phssthpok flew all the way from the galactic core on manual, without so much as a rudimentary autopilot. They just don't trust automatics, yet the ring is completely automated, from the flup dredgers and recycling system, to the attitude controll jets and the meteor defense laser.

5. Why only build one ringworld? When you finish one, why not just build another, and another, and another. You've got something better to do? Suppose there are other ringworlds aout there, would they let this one alone, knowing it's packed with a trillion potential protectors of radicaly different bloodlines?

6. Who keeps killing off the protectors that sporadicaly get created on the ring? Somebody must be doing it. Why are there any attitude controll jets still in place? If I were a new protector on the ring, the first thing I'd do it put a bunch of juvenile breeders of my species on an attitude controll jet ramship, with one adult and a time released store of tree of life. Launch the ramship and let the timer release the tree of life and turn the adult into a protector. When he wakes up he's already on a fast trajectory away from the ring. His mission is to establish the species on new worlds away from the ring, helping guarantee the long term survival of the species, whatever happens on the ring. It's simply the optimum survival strategy. (Your mileage may vary on this, any other ideas?). 

The Tnuctipun plot to follow shortly but you can find links to both the above and the Tnuctipun Plot on the Links page of this website.