The TB and TT do not have cabs available for them but they are price tractors and a cab wouldn't really keep them cheap.
As for the TC cab, NH has the means but maybe their research has shown that there wasn't as big a market for cabs on these machines. Think about it, almost everyone who has a TC either has the rops down for mowing or to get in and out of the garage. The limited few that would want the cab are most likely only a very small percentage and Laurin (I think that is how the name is spelt), makes a great looking cab for some of the TC's.
Has anyone e-mailed the New Holland rep and asked them their thoughts on it? Never hurts to toss an e-mail out.