I today played several matches as a Juggernaut and had over 15000 credits, but I still wasn't able to buy Shrike. According to Wiki, it costs 4500 credits, so I had more than enough. It allowed me to buy Grav Cycle and Beowulf, but not Shrike. Other players on the same server were able to buy that vehicle, so it wasn't disabled by the server. Why couldn't I buy the Shrike? Is there a limit of number of Shrikes that can be in a game at the same time?
So it seems heavy classes cannot buy Shrike. I switched my class to Soldier and went to inventory station, and then I was able to buy Shrike.