Why?
They still make money.
This.
Fallout 1 and 2 still make money. The thread's starter is portion of evidence to that. People like the fallout series, so they want to play the original. No reason to make it free if it's still a product a number of people are willing to buy and selling it outweighs letting it loose.
Hell EA made the orginal C&C's free.
Yes, I just said EA and Free.
In this case, I would say EA does so, because it works in the opposite direction, to help sell the newer CnC's. Also the CnC series is essentially and decidedly completed, unlike the fallout series.
The potential profits from the other older CnC's, aren't high enough to market them out any more; sells aren't high enough from retailers to market them on the shelves in place of other games, and other manners of selling doesn't bring in enough profit to interest them.
But, they are a product with very little cost to produce, in some cases just as little as the cost to run an upload server, so they can serve as a potential sells gimmick for the other games, by bundling them in with the other games, or as a way to advertize the newer more sellable portions of the series. Making the older games free, is essentially free advertising.
There's a reason you can buy a pack with basically every CnC game in it, on a store shelf; CnC is a near generational product that works both ways. Somebody that liked CnC ~8 years ago and downloads it now could end up buying the newer product. Newer customers might become intersted and want to try the older versions, and that works toward them wanting to complete their series collection. So, they buy that bundle on the shelf.
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I would say that in the end, the older fallouts will become a free download. But the current situation is doing too well for it to be considered just yet.
I predict that Fallout is going to continue for a while as a series if the companies involved have any sense; there's a buyer base for it.
Hope they also have sense enough not to fling out a shoddy rotten egg for one short term gain and ruin that, though.