Copied from the wiki so grain of salt and all that:

"Ulysses was originally supposed to be a companion in the base game of Fallout: New Vegas, but was cut from the final version of the game and was changed to appear only in Lonesome Road. However, the playing cards that came with the Collector's Edition included Ulysses, despite him not appearing in the base game. According to Chris Avellone, Ulysses "was a complicated character in terms of some of the hooks into the storyline." Avellone teased Ulysses' eventual return in an interview with Lightspeed Magazine in November 2010, saying "maybe he'll come back at some point."[Non-game 6]

Ulysses was ultimately cut from the base game because his recorded dialogue was so large that it would not fit on the disk. Even when compared to the second-biggest companion in terms of dialogue and memory, Cass, Ulysses took up much more space. It was too late into development for them to selectively cut down dialogue, so the team decided to scrap Ulysses from the base game altogether. Had the size issue been recognized earlier in development, Joshua Sawyer stated that they likely would have found a way to keep Ulysses in the game. The loss of Ulysses from the base game was a difficult one, as Sawyer and Avellone both felt that the game needed a companion that was more sympathetic to the Legion."

I think the Disk size concern was for PC and Consoles at the time, it was still an issue in 2011 where cinematics and such could take too much of a disk, and I think I remember a few PC games still releasing with multiple disks roughly within a few years of New Vegas, like Final Fantasy 13 has three disks I think for Xbox and PC? Nowadays not as much of a concern at all.