It's not an in-universe answer, but there's some interesting background here. The original pilot did not include Doctor Smith or the Robot. This included the sequences that were later used in early episodes where they took the trip in the chariot to a warmer climate. In order to use that footage, they had to come up with a reason Doctor Smith and the Robot did not go with them on that trip.
When Doctor Smith was added, the original intent was for him to be in the first few episodes as a saboteur and bad guy, but that they'd kill him off or he'd end up left behind or something like that. That's why he was originally credited as "Special Guest Star." This was also when Lost in Space was airing opposite the 1960s version of Batman from January of the 1st season of Lost in Space onward.
The tone for the 1st season of Lost in Space was black and white and started with a more serious and darker tone. Batman was in vivid color. To combat that, Irwin Allen, the producer, guided the show to a more and more campy and humorous tone. (And went to color, with an emphasis on bright colors in the 2nd season.)
Jonathan Harris was still, during the first season, not in a secure job, since his character was supposed to be written off the show, but Harris, of course, wanted to stay, so he worked hard at every chance to change his character from a dark heavy to a comical and likable trouble maker. (This included coming with, on his own, all the names he called the Robot, like "bubble headed booby.")
This fit in well with Irwin Allen's plans to make the show funny (and by that time Star Trek had the serious science fiction audience) and to fight Batman with more of the same. One day Allen burst onto the set, pointed at Jonathan Harris and said, "YOU!" Everyone looked at Allen and Smith and Allen said, accusingly, "I know what you're doing!" then said, "I want more of it!"
For an in-universe answer, while Don West would have gladly gotten rid of Smith, at least at first, even if he was evil (as he played it at first), the head of the expedition was John Robinson and he would be directly, on a personal level, responsible to his wife and children, and if he killed Smith, or let him die, or marooned him somewhere, that would have turned him into a murder in the eyes of his family. Maureen, on the other hand, repeatedly demonstrated a high level of compassion for Smith and actual enemies. This was the 1960s, and the image of a woman as gentle and compassionate, especially in a mother, was expected.