Why do all of these major sci-fi/fantasy franchises ALWAYS have to have a PREQUEL series??

Short answer is "money" and "ratings". It's a great time to be a television viewer right now because there's so many different shows to choose from. There's tons of competition going on right now.

Personally, I prefer movies over TV shows but occasionally you'll come across one that's quite special like Game of Thrones. We'll see if this HOTD series is any good but that's another reason why we're seeing them, they're riding the coattails of the success GoT, LOTR, SW and trying to increase their number of subscribers now that Netflix is bombing.

But I'm sure you knew all this, right?

My guess... prequels have established characters or lore that can be mined with new and cheap faces. If you go forward sometimes you have to pay someone like Han Solo $20 million and build a new story from scratch off a story that has already ended.

https://gifdb.com/images/thumbnail/make-it-rain-captain-swkn7kbxsd73iumf.gif

Yeah. The boring and wholly accurate answer is money.

There is another, more charitable answer though. You'll find even many relatively unsuccessful sci-fi/fantasy series follow the same patterns of having sequels, prequels and spin-offs. Even vanity published stuff in those genres. It's just part of the deal.

And it's because authors who world-build in this way enjoy playing around in the worlds they create and continue to furnish those worlds. Once you start creating a universe, it might be difficult to stop.

But, even with that in mind, the accurate answer is still very much: money.

Prequel is a way of saying 'I got no ideas left at this point moving forward, so I'm just gonna expand on some crap that was briefly mentioned in the original'.

And you are absolutely right - as far as track record is concerned, prequels appear to be even worse than the shitty (usually) sequels.

Money issues aside, it's actually easier to make a prequel than a sequel to some massive save-the-world Fantasy genre saga like this one. As with Tolkien's saga, all the magic is fading out of the world, there will be no more Others or Dragons or Giants, from now on it's only going to be humans acting like humans. So really, stories set in Westeros after the War of Five Kings and Two Queens are no longer going to be Fantasy genre stuff, in order to capture the Fantasy element the writers need to go to Westeros's past, when the world was full of magical/supernatural stuff.

And this is why the new "LOTR" show is a prequel as well, even Tolkien himself couldn't write a sequel to "LoTR". And this whole saga is massively influenced by Tolkien, everything GRRM wrote was either a copy of Tolkien, an attempt to be the Anti-Tolkien, or a parody of Tolkien.

Doing prequels isn't something new it's been in literature and film for a long time. It's just another way of making money of a successful story/product.

Just you wait till they come up with a preprequel.