I guess it is because at first most of the cars (or all of them) are blocked in order to force you to improve your skills.. You probably will be able to drive some Minardi (I haven't tested the mod myself) so you can increase your experience points. Each car has a number of these points, when you reach to it you will be able to drive it.
I don't know if it is your problem... but I thought it might help.
In fact I still haven't tested F1XR mod.. so I don't know what I would find on it... I was talking about CTDP 2005, in which I have begun with Minardi cause I have no experience and cannot choose any other car (I want to try without the cheat ISI_BABYFACTOR first until I cannot wait anymore to drive a McLaren or Ferrari )
Is Not only a problem of licensed copies. The sounds of F1XR06 are encrypted by ISI but after the upgrade of the rfactor.exe to v1.255F this encryption doesn't works anymore. The result is a mod with no sounds on the v1.255F.
The solution is to uncrypt this sounds. I know how and I have the tools but I have no contact with the author. He seems disappeared.