Ixion

It is unknown why Ixion was never released by SEGA, but it was around this time that SEGA Enterprises (the American arm of SEGA) was sold to Bally. After this sale, all of SEGA's 2600 titles were Bally/Midway arcade games (Tapper, Spy Hunter, and Up 'n Down). It may have been felt that only well known arcade game would sell well enough in the overly competitive 2600 market, or perhaps Ixion simply got lost in the transition shuffle.