As mentioned:
It works well.
It's uncluttered.
The second being THE killer selling point IMHO. Google has been around for a while, and it gained popularity at a time when most people were using dead slow modems. Waiting half a minute to have crap (read : advertising, news you aren't intersted in) load was only going to make people flee to another uncluttered search engine. And even today with a fast connection you can still enjoy the uncluttered interface.
It works well.
It's uncluttered.
The second being THE killer selling point IMHO. Google has been around for a while, and it gained popularity at a time when most people were using dead slow modems. Waiting half a minute to have crap (read : advertising, news you aren't intersted in) load was only going to make people flee to another uncluttered search engine. And even today with a fast connection you can still enjoy the uncluttered interface.