I wouldn't agree that Crimson was a big team pack. I don't think I'd agree that Castle is either, having played a bunch more of those maps (despite whatever messaging they're putting out there). Of Crimson, only Shatter was really BTB focused. The other two, while mediocre, were sized for 10 or 12 people. Not ideal, but also not surprising given the way they've been inflating player counts for existing modes over the years. With Castle, Outcast seems to be the one that most benefits from 16 players. Daybreak plays better with 8 or 10 people than even 12, so how much of a disaster would 16 be? It's not a large map, or at least doesn't play at all like one. Perdition is somewhere in the middle but I'd guess that 16 people in there wouldn't play as well as the 12 it's using now. If they were serious about pushing Castle as a big team pack, then it probably would have been integrated into BTB when it launched.
The real issue isn't that they're only making Big Team maps or that they're not making maps for 4v4, it's that they're trying (and often failing) to do both by making 6v6 maps that don't really satisfy either camp. It's been a problem with Halo for a while now, maps that straddle the line like that don't usually turn out very well. Something like Standoff is an exception.