Here's a possible explanation: They didn't specifically target Bosma. They had a plan to execute the owner of a 3500 truck. Around the end of April, Millard had some urgency to finally carry out the mission. He needed the 3500 truck as soon as possible (perhaps due to an upcoming race that he wanted to go to). He was pushing Schlatman, Jennings, et al to finish building a trailer as well.
Come the weekend of May 3, Millard was fixated on getting the mission done no matter what. He had attempted to make various preparations (such as getting the generator connected to the incinerator), but he didn't quite have everything in place. He was stressed about his cash flows, the business accounting, the property deals, the several women he was juggling, etc. He wasn't thinking straight. But he needed that 3500 right now or it would spoil his trip that he had planned.
The night of Bosma was the last chance. The truck was perfect. The circumstances seemed pretty good. It was nighttime, there weren't many people around. They could murder this guy and have the evidence cleaned up by the following day. They seized the opportunity and improvised from there.
Of course, nothing is ever as easy as one thinks it's going to be. There was way more blood than they expected. They accidentally shot out the passenger window. They were running on adrenaline and possibly stoned or high on cocaine. The generator wasn't working with the incinerator. Maybe they forgot fuel for the propane tank. Lots of unexpected things could have happened. They had to make decisions on the fly.