Actually, we don't know.

We don't know how they selected sites for major buildings and temples -- unlike some cultures, there's no history of astrology until the Ptolomies, some 2000 years later (where we could presume that there's an astrological reason for the alignment.)

Nor do they seem to have used divination heavily.

I think part of it has to do with the lessons they learned from building some 60+ pyramids before starting this one -- and it's being the capital city at the time.

But you'd have to know what that area looked like when they considered it for a building site. Was it grassy or sandy -- where was the Nile in relation to it (not where it is today, that's for sure!), and so forth. They knew they'd have to move a lot of stone and that they used the river and barges to transport many large and heavy objects. There's also the significance of the desert and its relationship to jackals, convenience of worship (they built their mortuary temples in and close to their capital cities), and so forth. Distance to the quarry and local quarryable limestone was also undoubtedly a factor.

So I don't think there's just one reason... there are many reasons.

If it was a Hindu or Chinese monument, then divination and/or astrology would have played some factor in the placement.