The monolith orbiting Jupiter could presumably have imploded the planet into a star at any time in the last three million years. Why did it wait until the Alexi Leonov was in imminent danger to start the process? Why not three million years ago? Or ten thousand years ago? Or in 2002? Or a week later when nobody would be in danger? Was endangering people part of the decision process? If so, why?
There is a brief (but inconclusive) discussion on this issue within the film's tie-in novel; 2010: Odyssey Two.
‘I’m sure you will, Vasili,’ Floyd agreed. ‘But there’s a more important question. Why did they do it?’
‘A warning?’ ventured Katerina over the ship’s intercom.
‘We’ll find that out later.’
The implication seems to be that the Obelisk's reasoning was twofold;
Having detected the probe in close orbit to Europa, the Obelisk decided that the time was right to defend Europa from further intrusion (hence the destruction of the probe and the warning message relayed by Hal to Earth).
Having detected that humanity was now capable of space-flight, the Obelisk decided that the time was right to provide us with two additional habitats that could be immediately colonised. Having done so earlier could have potentially had the opposite effect, encouraging us to remain on Earth.
On the credit side, the end of night had vastly extended the scope of human activity, especially in the less-developed countries. Everywhere, the need for artificial lighting had been substantially reduced, with resulting huge savings in electrical power. It was as if a giant lamp had been hoisted into space, to shine upon half the globe. Even in daytime Lucifer was a dazzling object, casting distinct shadows.
Farmers, mayors; city managers, police, seamen, and almost all those engaged in outdoor activities - especially in remote areas - welcomed Lucifer; it had made their lives much safer and easier. But it was hated by lovers, criminals, naturalists, and astronomers.