The problem is that, from their point of view, they put money into getting the Mosler into the game so they need some sort of return for that. Keeping player numbers high week after week is clearly important to them (and the shareholders), but putting it in the Forzathon shop means you don't have to do much to get it, meanwhile making it the trial reward means the average player would probably need to try a few times to finally win it - that does keep players playing longer without forcing it too much (like, say, earning several million skill score in a car that won't even drift without a lot of modification), but is chronically not fun for basically everyone.
They've struggled to find ways to keep us engaged throughout the week, the earliest example of that was the 7 million skill score in an offroad car weekly Forzathon challenge. They don't want us to grind through that stuff in a single sitting but we don't want to actually put time into stuff, especially when it's
that boring and, apart from the 14 skill points, you don't get anything until it's finished.
It's a difficult thing to work out because they throw cars at you, get you accustomed to it and then have to incentivise you to keep playing - once you've collected all the cars, how do they do that without giving you more free cars? At the same time though, if they artificially locked more cars away to give out later on, people would be just as annoyed by that.
So I can see why they did it this way, but it's not a good solution to what they want to achieve because it just annoys people.