Also, Pacific Blue they released the complete series set the same day as season 1. I assume the complete series set sold better then the S1 set so why would they bother releasing the remaining seasons when most people who wanted that show probably bought the complete series set?!
I wouldn't be surprised if Mill Creek already knows that most of their stuff ends up in the dump bins relatively quickly, shortly after release. For tv show season sets, perhaps Mill Creek has adjusted their marketing strategy based on this premise.
Over the last few years, I frequently saw many of the individual tv season sets of various Mill Creek shows in the local $2 dump bins, such as: The Commish, 21 Jump Street, Renegade, Hunter, etc ... But the "complete series" dvd sets don't show up as often in the local bargain bins. (They occasionally show up at Walmart, such as the complete series sets of 3rd Rock From The Star, That 70s Show, Roseanne, etc ...).
One possible marketing strategy is to use the season 1 (or 2) sets found in the $2 bargain bins, as a way of enticing possible customers to find and buy the more expensive complete series sets. I don't know how effective such a strategy is.