Why does this series lack continuity?
Series 1 and 2 were done on a shoestring or on the cheap, they came to the conclusion that Lister was travelling with Rimmer and Cat, who would eventually have two children, Jim and Bexley.Series 3 reintroduced a minor character, Kryten, to give it a more rounded feel. They made Hollie a female (due to Norman Lovett leaving). It also looked a bit more expensive.Series 4's White Hole was a temporal anomaly and therefore it was meant to lack continuity.Series 5's Back to Reality created a fake reality. Series 6 creates a pocket reality where they discover a time machine.Rimmer wanted to leave the show for most of Series 7, so he was replaced by Kochanski. Due to the original Kochanski not being a good enough actor (apparently) she was recast. For some reason when Red Dwarf reappears Hollie was played by Norman Lovett.This was more or less a reset button, meaning the crew was revived, but there were now improvements on the ship, as well as a prison section. Unfortunately this series ends on a cliffhanger.Because they wanted to get rid of the crew again for the new special, it was said they just wandered off. Another alternative reality was created when Red Dwarf had apparently become a sitcom.In Series X this status was more or less the same.The simple answer is that it's sci-fi, so they can do what they like.
When Nova 5 crash landed and the ship's crew died, why didn't Kryten leave Nova 5 and save himself?
Kryten is a mechanoid, a robotic butler, who was created and programmed to serve and obey humans, and was loyal to the crew of the ship Nova 5. Stranded aboard the Nova 5 for many centuries, Kryten never left the ship and the human skeletons of the members of the dead crew. Due to his programming he continued his duties aboard the Nova 5 and never attempted to save himself. Kryten had very much withdrawn into a fantasy in which the crew was still alive. He put wigs and clothes on their skeletons and made them their meals and served them tea. In his off time he settled down to watch his favorite soap opera "Androids" and dreamed of having his own garden. Kryten did send an SOS distress call which was only answered centuries later by his next shipmates from the Jupiter Mining Corporation mining ship Red Dwarf.
Ace Rimmer is an alternate version of Arnold Judas Rimmer from an alternate reality. In Ace's dimension, Ace Rimmer is a selfless, brave, flamboyant and likeable test pilot in the Space Corps and has the rank of commander. Bongo, Ace's superior officer, assigned Ace to test pilot a new space vehicle that can travel across dimensions and would allow Ace to meet many different versions of himself from those dimensions. Ace arrived in *our* dimension and accidentally collided into Starbug, causing it to crash on a watery planet. Ace came to the Red Dwarf crew's aid and met alternate versions of his friends in the Space Corps: Dave Lister (Spanners, Space Corps engineer), The Cat (Padre, a priest), Holly (Melly, Bongo's secretary) and Kryten (Bongo) and Arnold Judas Rimmer (himself), who had bitter feelings towards his alternate self. After successfully aiding Lister in repairing Starbug and surgically repairing Cat's damaged leg. Ace decided not to stay on "Red Dwarf" and continued his adventures across the dimensions and meeting other versions of himself. But tragedy later struck, when Ace was killed, when an neutron tank exploded in dimension 165 and since then, alternate versions of Ace have assumed the mantle of Ace, becoming a swashbuckling James Bond-like space hero. Many versions of Arnold Rimmer have become Ace, recruiting and passing the role onto the next. When the latest Ace's light bee was damaged by a gunshot, when rescuing Princess Bonjella from the Nazis, Ace returned to our dimension and arrived aboard Starbug to recruit and train our Rimmer and revealed to our Rimmer, that he is also a hard light hologram and Ace was dying from a power leakage, as the result of the damage from his light bee. After Lister dressed up as a knight, tricks Rimmer (As Ace) into fighting him to a duel and shooting him with blanks from a Bazookoid. A proud Ace soon died and Lister fooled Kryten and Cat into thinking their Rimmer had died and that their Rimmer was Ace. After Rimmer's funeral and following Ace's coffin to his final resting place, in orbit around a planet, which was also the final resting place of all the other Ace Rimmers. Rimmer left Starbug and begun his adventures as Ace. When Red Dwarf was slowly being disintegrated by a microbe, Ace came to the resurrected Arnold Rimmer's rescue and in return, Hologram Rimmer got the Ressurected Rimmer to take over as Ace and Rimmer rejoined the Red Dwarf crew.