Completing Sets

[trey]

So, there's not many titles I tend to skip picking up. Honestly its become a bit of a compulsion. Though there are some titles I don't pick up because the come in the middle of the series and I haven't had the opportunity to catch the beginning of the series. I realize I could come in part way, but in the forum of purchasing series (tv especially) i'd rather start at the beginning. Also to some extent when its part way through you know at some point there will come a set.

I can't remember what made me think about this a few times in the past, but why do some studios only put out upcoming series as opposed to some of both. Smallville still doesn't have the first few seasons (grant it they're not really that great, but still for the purist), Lost finally came out with the first two, but for quite sometime you would have to come in on the middle. Supernatural and sopranos are a coupe others.

Why do you think studios don't come out with those previous seasons? slow sales? focusing on newer material? don't think people will double dip?