It seems mostly a combination of inexperience with such situations, panic and bad luck. After the Alien births from Kane, the crew split up into two groups of three. We follow Ripley, Parker and Brett as they search for the Alien. They come across Jones the cat who runs off. Parker tells Brett to go get Jones so they don't risk picking it up on the motion tracker again. This ultimately leads to Brett's death at the hands of the Alien. But keep in mind that the crew were still expecting to find the Alien when it was in its chestburster stage. They could not have anticipated the Alien growing to over 7 feet tall in a matter of hours.
Next, Dallas goes into the air ducts in order to try to bait/flush the Alien towards the airlock in order to suck it out into space, leading to his demise. This was likely because the air ducts were so small and cramped that having two or 3 people would slow them down greatly and cause more problems than it would solve, including making too much noise. Furthermore, there may have been only one functional flamethrower, and since they cannot use guns due to the risk of the Alien's blood burning a hole through the hull, sending in more people without a weapon would leave them defenseless against the Alien. And even if there were more flamethrowers, it would put them in danger of hitting each other. They may not have wanted to risk too many people against an unknown life form anyway. The idea wasn't bad, but unfortunately, with Lambert crying, shrieking and panicking for Dallas to get out of there into his earpiece he understandably moved quickly, without thought and wound up running right into the Alien.
Shortly thereafter, Ripley asks Parker to refuel one of the flamethrowers. Though she tells Ash to go with him, Parker refuses (seemingly because he doesn't like/trust Ash). There is a brief scene that shows Parker moving cautiously through the ship to get the fuel. He manages to get back without incident.
Finally, Parker and Lambert go on together to collect supplies, as Ripley goes after Jones the cat on her own. Ironically, it is not the solitary Ripley who is singled out by the Alien, but Lambert and Parker. They may have thought that, since the Alien had restricted its activity to the lower deck and places with access to the ducts, Ripley would be safe on the upper deck anyway; and since it always targeted people who were alone, Lambert and Parker would not be in danger in the supply room as long as they were together. Unfortunately, the Alien used the fact that they were extremely hurried and neither of both was keeping watch to its advantage, by sneaking up on Lambert; and its close proximity to Lambert prevented Parker from using a weapon.