Very good, interesting question:
The obelisk form is indeed ancient and ubiquitous - pre-Columbian American civilizations, ancient Egypt, Asian civilizations of long ago antiquity all were enamored with the obelisk and placed it at the center of their ritual and symbolic places of gathering. (See Paul Devereux's works)
Some have postulated that this indicates a common root for all human civilizations. The Talmudic sages believed that all civilization emanated from the dispersion that according to the biblical account occurred after the destruction of The Tower of Babel, which appears to have been an obelisk with a shrine at the top, according to Medrashic and other ancient sources (which I do not recall at the moment). However, modern archeology and anthropology seem to refute this idea - we find the obelisk form at sites which apparently pre-date what would be supportable according to the biblical timeline, in Sumeria and South America for starters.
IMO there is something about the obelisk form that inherently appeals to us humans. Many of our modern skyscrapers also emulate this form - the new WTC in NYC is essentially an obelisk, and I don't think it's simply an imitation of classical style.
Perhaps the obelisk form resonates on a deep human level: it evokes a feeling of looking-striving-surging upwards - so fundamental to the human condition. Certainly the new WTC reflects such feelings, as did the very ancient biblical Tower of Babel.
In addition, (thanks to Eugene Seidel for this edit...) a high place to give a strategic advantage in warfare, and political motives - the expression of power and might - were also very important - this is documented: (Again, see Devereuex) Rulers would appear at the top of the tower to demonstrate their closeness to the Gods and reinforce their god-like stature to the populace.
Still the question returns - why is this form linked to such feelings. (Many obelisks contained nothing, or simply a shrine at the top.) And the effort and great emphasis on esthetics in these structures indicates that they were about more than just a high point to gain strategic advantage in warfare, etc
Bottom line: obelisks are attractive and interesting everywhere, to everyone - as antiquities and mementos or as modern day manifestations of ancient designs and motifs. Certainly there is a particular history surrounding each obelisk - how/why it was built or got to be where it is. But I think my explanation is the fundamental underlying answer to your question.