Why was it so obvious to Newton that planetary orbits should be elliptical?

Why is it obvious to you that the speed of light in a vacuum is constant?

The reason is others have done all the ground work. In the same way Newton didn't have to figure out that the orbits of bodies were elliptical - others like Kepler and Copernicus had done the ground work. Newton could build on Kepler's work like Kepler could build on Copernicus'.

Newton himself said "If I have seen further it is by standing on the shoulders of giants."

Any trajectory in a spherically symmetric inverse-square force field has to be a conic section.

(This was the essence of Halley's question to Newton).

The general case for a closed trajectory is an ellipse. A circle is the special case when the conic section is parallel to the base.

Kepler figured it out way before newton.

Because nothing so perfect as a circular orbit could exist.