Probably some badass alien invasion that's forty years(?) away is too abstract to convince a bureaucracy-choked Republic to get off its ass and react, while an "enemy at the gate" is going to stir enough influential senators into action and a belief they need to turn to militarism after a millennium of pacifism.
You say that like they had much of a choice.
Palpatine didn't turn a Republic into an Empire with some made-up boogieman or the faint threat of fringe insurgents. He did it by promising people a bright new future. Safe, secure and free of the decadence and corruption of the old decaying Republic.
If anyone feels this is unrealistic, I'd invite to pick up a history book covering the early 20th Century. There's more than a few examples of just this sort of thing and I'm not just talking about Germany in the 1930's.
From some of the new books that have come out post EU purge, they've hinted that Palpatine was playing a longer game than "Kill all Jedi. Become Emperor. Work on cackling skills."
Indeed, they've portrayed him as a very distant, uninterested ruler. Hardly seen in public, or even within his palace (previously known as the Jedi Temple, which I thought was a nice touch.) Indeed, he seems more interested in some ancient Sith site buried deep beneath the old temple, which leads me to speculate he was going after his old master's goal of conquering death.
It seems perfectly in character that he had no intention of passing his knowledge and power onto his apprentice. And that he fully intended to outlive them all. it ties into the notion that the key difference between the Jedi and Sith (perhaps even the cause of their ancient schism) is that the Sith don't buy into the netherworld and the cosmic force. That oblivion is oblivion and they'd do anything to avoid it.