Do you have any information why they switched for Civilization V? I tried to google it but so far only stumbled over flamewars regarding various scripting languages without the Firaxis' reason

Its hard to give you firaxis' reason (unless one of their employees lurk around here), but there are many reasons to choose lua over python.

The main advantages i can think of are:
1) Lua is smaller and faster. (at basic arithmetics and function calling, it loses greatly at tasks which the python standard library handles)
2) Lua has a slightly simpler syntax (no strict indentation) which makes life easier for modders, allthough i guess those who modded Civ4 are unhappy about the change as they most likely get less flexibility and have to learn a new language).

If they wanted people to write Civilization mods that turned the game into a first person shooter then yes, Python would be a superior choice, but for setting a few values and flags at runtime it just doesn't make much sense.

I think it was due to speed. One of the original complaints about Civ 4 was it was too slow. Both Python and Lua are equivalent high level languages so flexibility wasn't the issue, imo. Lua in general runs about 4-5x faster than Python..

Sounds logical, thanks.