Well, one answer is that US hydrophone technology was probably superior at the time - but that's not necessarily a well-established fact, mostly an assumption. Still, it would stand to reason. SOSUS benefited greatly from cutting-edge research into acoustics that Bell Labs had been performing for other reasons, the Soviet Union probably didn't have the hydrophone technology or the undersea cable technology it relied on.

There's a more interesting answer if you want one, although this is decidedly a conspiracy theory with, I would say, "medium" credibility within the realm of conspiracy theories. Some believe that both K-129 and Scorpion were sunk by enemy action, K-129 having been sunk by an accidental collision with the Swordfish and Scorpion having then been torpedoed in retaliation. The story goes that the admiralty of both countries, agreeing this situation could rapidly escalate into an undesirable war, agreed to suppress information on the cause of both incidents. The Soviet search for K-129 and American search for Scorpion could both have been cover operations.

Yeah, it doesn't make total sense, and the evidence supporting this theory is a combination of circumstantial and recollections of people in their 80s. Besides, in the later sinking of the Kursk, Russian leadership immediately blamed a collision with a US submarine. But obviously the Russian political climate of 2000 was very different from 1968. It's a fun conspiracy theory.

A more interesting conspiracy theory is that K-129 was on a rogue mission to launch nuclear weapons on the US and was torpedoed by the US (once again perhaps by Swordfish, it was in the right place at the time) to prevent this after being tipped off by by the USSR. If that sounds a bit like the plot of The Hunt for Red October, well, it does. The evidence for this story is not nonexistent but it's pretty limited, and no one takes it very seriously.

Still, it gets at one of the oddities of K-129: the Soviet Union searched for it in its assigned patrol area, but the wreck was ultimately found far away from its assigned patrol area. I don't think anyone has a really good explanation for this. It was not at all typical for Soviet submarines to go off on their own, Moscow kept very tight control of them. So it seems that either Moscow didn't know where K-129 was (perhaps suggesting some kind of plot, whether of defection or rogue attack who knows), or they knew where it was and searched elsewhere to avoid showing their hand (suggesting K-129 was on some sort of very secret mission). I tend to suspect the latter is more likely, K-129 may have been ordered to leave its patrol area and approach the US as a show of force (this happened at other points in the Cold War) and when it was lost the search was conducted in the normal patrol area to avoid revealing that had happened. All indications are that SOSUS was successfully kept secret from the USSR for quite some time, although certainly not all the way until 1991.

Tom Clancy seems to have based The Hunt for Red October at least in part on rumors about K-129. Yeah, I watched too many submarine movies and read too many submarine books as a kid. What can I say, I had a middle-aged father.