WHY do planets orbit the sun in an ellipse?

OK, I accept that Kepler calculated that the orbits of the planets are not circular, but elliptical. I accept that for bodies as close as we are, both foci may be in the Sun. What I don't understand is WHY the orbits are elliptical. The folks who thought circular weren't all that crazy. There appears, after all, to be only one large focus. Tether balls don't travel in ellipses, do they? I would expect an ellipse in a binary star system. So what's the other focus? The gravitational pull of other planets seems rather minor compared to the sun, but maybe that's it. My guess? Residual momentum from formation of the planets from gas in the first place is an ongoing vector.