Because it isnt "good"
Also what defines Old? gog sells tons of games I dont consider old.

They have. I can't provide you with a link, but the question has been asked before, and they've said that yes, they have declined some games.
As for "old", that's more of a guideline than a rule, I think.

Do you know some specific game names? They maybe happen to be some of my favourites...

To the best of my knowledge, they have not named any specifics, and I can understand why. It wouldn't be fair to anyone.

While GOG have declined to release some games, I doubt it was anything to do with them not being "good". After all, "good" is just as subjective as "old". It is more likely that games that have been refused are refused due to technical reasons or for contractual reasons.

Yes, sorry. I should have clarified that I only meant they have turned down some games. I didn't intend to include any assumptions as to why they did.

While GOG might focus on providing as many early PC era classics as possible, clearly they're open to any game of any age for which the publisher is willing to drop the DRM (as long as it's not complete crap of course). I think the reason behind that is simply a case of increasing their revenue as much as possible without sacrificing their core values.

Do you think GOG would accept REALLY old pc classics like Alley Cat? Me wants a legitimate copy of Digger. :(

They have 80s games here already. So I don't think it's impossible. But with two fixed price points, I think it would have to be in a pack or something to justify the cost.

Another problem is that the older the game is, the more diffucult it can be to find out who owns the rights to it.

It's as if games have their own family tree. The further back you are trying to trace it, the more difficult it becomes.

I wouldn't put it past them to refuse a "bad" game, be it because it has too many unresolved bugs or because most people simply didn't like it (daikatana style) and they wouldn't predict good sales for their efforts. Bear in mind that "didn't like it" and "didn't buy it" are not synonymous. Many of GOG's games are pretty obscure titles that failed to be a success at their prime for some reason or other.

I'd also wager some games were rejected because of being "bad taste" or unsuitable, like sex games and titles like "porntris".

They declined system shock 2 because they hate you.
I've discussed this with them, if there was demand for it on the wishlist they would gladly sell a game like that.