I'd guess because it's not quite as demanding, from the top of my head. I remember I bought The Hunger Games, finished it in two days, and found it ok. Then around a week later I bought Game of Thrones, and it was a completely different experience, then came Clash of Kings, Storm of Swords, Feast for Crows and Dance with Dragons. When that finished and I went looking for the second book after Hunger Games, I personally couldn't enjoy it because I found the characters to be too shallow, and the setting somewhat incongruent, although if I tried now I'd probably read it now that ASOIAF is not so fresh in my mind. Although I'd rather use that excuse to read ASOIAF again,
The thing is, ASOIAF is very complex and time-consuming. It took me three weeks to read it all, and I can read very fast and dedicate a long amount of time to reading, so I can see how it would take months for someone who's not as avid as me. Also, it has complex relationships, complex motivations, and portrays reality in a gritty and blunt way, something not everybody would like. And I must admit that there were parts that were not very exciting, I find it as a necessary evil because not all events must be like that, but still many could get bored.
Also, you can't take your kids to watch Game of Thrones
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To build may have to be the slow and laborious task of years. To destroy can be the thoughtless act of a single day. - Winston Churchill