Service localisation and user confidence.
It is entirely possible to automate the selection of the correct language etc when a visitor comes to your website, however:
a) This usually requires client (browser, screen reader, PDA... etc) header information which is not always sent, so a mechanism needs to exist to choose a fallback position,
b) It's a lot of work, and disproportionately expensive for small businesses, individuals, or non-profit organisations,
c) Studies show that consumer confidence increases when they perceive a service to be local or domestic, so why not use a localised TLD?