Pooh - it's weird with Pyramids because it's a really good book and I think it's interesting 'historically' as this is the first book where Terry's really getting into his best satirical style. By that I mean that you can tell its deliberately 'out of theme' and completely different to the other books that went before as there's no wizards, witches or even A-M except for the 1st bit with Teppic and even there he's doing his Assassin finals. So the Djelibeybi aspect is 'exotic' and it's really funny with the way Terry plays with time, religious observance and cycles and introduces Tsort and Ephebe in a more impactful way than previously.
And it has camels of course which nearly killed me as I happened to be on my umpteenth G&T when You Bastard first met my eyeballs and the spluttering was spectacular
Certainly Pyramids was my favourite Pratchett book at that point and by a long margin too, but there were so many great books that followed in swift succession that I think poor old Pyramids rather got eclipsed as time went on - I prefer Moving Pictures and Reaper Man, Small Gods and GG and Wyrd Sisters (although that was before of course but was also 'in the comfort zone' as it had the Witches so recognisably Discworld).
All down to personal taste again and 'better' and 'best' with Terry's books is almost impossible because the standard is always so high and each book really does have something outstanding to mark it as a favourite. I prefer Small Gods to Pyramids although they're both in essence about religion as a political weapon (well that's my take anyway) as SG does it more stylishly and probably because I'm an irrevocably lapsed Catholic so I'm more drawn to the issues in there and how they're dealt with than to Egyptian myth and cultural custom which I actually quite like reading about.
Remember you did say you wanted to know why - so there!