When an indie game goes on a daily deal, it is 75% off even if you buy it on its own. Older games, on the other hand, are only 75% if you buy a whole bundle of them; otherwise, they are 50% like the rest of the time during this promotion. Why is there such a difference? Is it because licencing issues? Classic games have lower margins? Is it just the way GOG likes it?

I am exploring adventure games for the first time and I would have gladly purchase some of these titles to see if they are to my liking. But the whole lot? No, thanks. End result? I'll probably buy more indie games than classic games during these sales. How about you?

I think it is because oldies have more games from the same devs/publisher while indies have mostly one game of the same dev.

Also, because most oldies are priced for 5.99. -50% is usually less than 75% for more expensive games.

Don't look at percentage, look at a price. IF you think 3 bucks for an old game is ok, just buy it.

Pretty much this. Even at 75% off many of the indies cost as much or more than the 50% off oldies.

I just bought Quest for Glory 1, 2, 3, 4 and 5, for 4.99 (US). That's one dollar(US) per game.

Growing up the best steal-of-a-deal we could get for games was to buy them in more-or-less clearance from Electronics Boutique for ten dollars. We didn't get a choice of game, we just got to leaf through the ones they were trying to get rid of mixed in with the others. If you spent 20 minutes you could maybe find 5-6 options, usually 1 of which you'd ever heard of, and most of which looked awful.

We'd take a chance anyway just for the fun of seeing what it was like. They were usually horrid.

This doesn't even get into the change in money value, where you can multiply those numbers by nearly 2.

Music is still over ten dollars an album. Seeing a movie in the theatre is more expensive than ever. Games, especially on this site, are megacheap, and the sales are even cheaper. It aint no thang if you don't get 75% off on whatever you wanna buy.

First off, 50% is usually a good deal as well, although I admit 9.99$ for KQ1-3 is a bit high.
The oldies are so far by the same publishers, the others are not, maybe it has something to do with that.
And those bundles usually have a few games, that are not so sought after. Those wouldn't gather too much interest even a 75% off, but are gladly bought in a bundle.

And if you want explore adventure games for the first time, better don't start with the early Sierra games. They are pretty unforgiving with their random deaths and dead ends. And those that use a text parser are much too difficult for me.

The oldies tend to be groups of games from a single publisher. I wouldn't be surprised if the deals with those publishers state they can only give a bigger discount if the games are bundled in a pack together.

With indies, and sometimes other single games like Witcher 2 and Riddick, the company doesn't have enough individual games to do a full pack at once. So they can do the larger discounts without requiring a full bundle.

Edit: Also Square's games go off sale right before the Steam sale starts. I'd guess they don't want games on sale at both places at once, so they are making GOG end the games early.