I don't remember all the details, but there was some convoluted explanation as to how they were preserved, and why they couldn't be revived, and Superboy must leave their bodies to roam space through eternity
here are the details thanks to darkmarks site (
http://darkmark6.tripod.com/superboyind3.htm)
Superboy No. 158
July 1969
Cover: Superboy looking at space capsule bearing bodies of Jor-El and Lara //Neal Adams
Story: “Superboy’s Darkest Secret” (24 pages)
Editor: Mort Weisinger
Writer: Frank Robbins
Penciller: Bob Brown
Inker, colorist: Wally Wood
Feature Character: Superboy
Supporting Characters: Jor-El, Lara (as bodies and in flashback; last chronological appearance; see Comment under MORE FUN COMICS #101 for chronology), Jonathan Kent, Martha Kent
Intro: Dr. Krylo (dies in this story)
Villains: Xonar (formerly Khai-Zor; last appearance in ACTION COMICS #223; dies in this story), Val-Arn (in flashback; last appearance in ACTION COMICS #223; dies in this story)
Comment: This is one of the most controversial Superboy stories of all, in terms of its “canonicity”. Since the bodies of Jor-El and Lara are shown in this story, with the implication they could someday be revived, many Superman Family scholars deny its inclusion in the Superman canon. Also, Krylo’s presence during the launching of Kal-El’s rocket has never been confirmed in any other text. However, we have chosen to include it as a possibly canonical story, especially in view of the presence of Val-Arn and Xonar, who also appear in a canonical Superman Family story.
Jor-El, in this story, says his transmission will start 15 years after the destruction of Krypton. This is probably inaccurate, as Superboy is probably not yet 18 years old; most likely, it is 12 or 13 years since Krypton’s destruction. (Also, no notation is made about the differences between the Kryptonian and Terran years’ length.)
Synopsis: Superboy receives a radio message which comes from a recording made by his father Jor-El. He gets a fix on its location in outer space, discovers the bodies of his parents, Jor-El and Lara, in a floating space-capsule “coffin” radiating Green Kryptonite rays. A Kryptonian villain named Xonar, formerly Khai-Zor, an enemy of Jor-El’s, almost knocks him into the Kryptonite with shaped charges, but another Kryptonian, Dr. Krylo, rescues him.
Krylo explains that he had believed Jor-El’s prophecies of doom for Krypton, developed a form of suspended animation which he hoped would preserve himself and Jor-El’s family in space “coffins” to be flung into space by the explosion, with tape-recorded instructions on revival procedures included. After Kal-El’s rocket was sent into space and Jor-El finished his last journal entry, Krylo ray-gunned both Jor-El and Lara down, subjected them to suspended animation treatment, and placed them in a space capsule equipped with a taped message for Kal-El, to be activated when he came of age. Seconds later, Krylo’s robots did the same procedure for him and placed him in another space capsule. Both capsules were rocketed into space only minutes before Krypton exploded.
Both capsules were found by Xonar, formerly Khai-Zor, whose partner Val-Arn had perished in the cataclysm; Xonar himself survived Krypton’s destruction in a rocket he had built similar to Jor-El’s plans. Xonar revived Krylo and used the taped message of Jor-El’s to lure Superboy, son of his old enemy, into a trap. In the conflict that follows, both Xonar and Krylo die, but Superboy manages to get Jor-El’s and Lara’s capsule back to Earth with the help of a space-suited Jonathan Kent.
But Jor-El’s tape is activated shortly thereafter and plays all the way through. The voice of Jor-El tells his son and his son’s foster father that Jor-El had been irradiated with killing radiation while on an expedition into Krypton’s interior, which radiation he passed along to Lara, inadvertently, but not to Kal-El. Since the incurable radiation would doom them both to a slow, lingering, painful death if they were revived, Jor-El asks that Superboy leave them both lifeless in space, should Krylo achieve his objective of preserving them against his will. Sadly, Superboy and Jonathan Kent return the capsule to outer space.