Speaking as a trucker myself, I think they underestimated the task they were trying to automate and at least some of the investors started to realize that. Allow me to explain: trucking, despite how brain-dead simple everyone thinks it is, isn't some stupid, monolithic field. There are very different sectors of trucking, with very different requirements of its practicioners - and, as a result, one software suite won't rule them all.
Allow me to elaborate: I currently haul privately-owned automobiles cross-country for owners who (for whatever reason) either can't or won't make the drive themselves. I pull an 8-car trailer (that is, a trailer with a capacity for 8 standard-sized autos). This means actually I drive the semi to your house or other location; load the car by driving it onto the trailer (it should be obvious how those cars get onto the trailer but, trust me, a lot of people have never even thought about it); securing the vehicle by strapping the tires to the deck; and rearranging the vehicles such that the semi doesn't exceed federally-mandated gross and per-axle weight limits, while still somehow relocating the vehicles so that the first vehicles off are the last loaded. And doing this 7 more times, at 7 more locations within 2 states. And, once loaded, I get to reverse that process 8 times at the other end of the U.S.
Driving is actually the easy part of this job. But (and this is the point I'm clumsily trying to make) it's not the only part.
Then there's the fuel haulers. The oversize/overdimension freight haulers. And what about hazmat? You really want to share the road with some robot hauling nuclear waste? Trucking is not just some redneck yahoo dragging a box down a four-lane interstate and I believe some people's hubris absolutely blinds them to this fact. There's a lot of things going on that no one's been able to automate (yet), and the only "androids" we have now can only show us Tik Tok videos.
Oh yeah, there's also this to chew over: why is it that the only successful tests of these automated trucks seem to be on four-lane highways (or ones with nice, wide shoulders); with clearly-painted lane markings and fog lines; and during fair weather conditions? Ever see one of these tests in lake effect snows? How about during one of these Biblical-level, apocalyptic thunderstorms we commonly get here in Florida during the summer? I'm betting some of these investors eventually started asking themselves these same questions.