I don't understand why it took so long to seriously considered the possibility that orbits are ellipses.

It seems that a circle is the simplest, most natural, most elegant hypothesis for the shape of an orbit, and an ellipse is the second-most simple/natural/elegant hypothesis. But instead of checking if an ellipse fit the data, everyone settled for 'a lot like a circle, but you have to include a bunch of fudge factors to match the observational data'.

Apparently Kepler had a similar view. April 1605 is when he figured out that the orbit of Mars was an ellipse around the sun; two years earlier when he was already in the process of trying to figure out what sort of ovalish shape fit the data, he said that he had considered and rejected the ellipse hypothesis because if the answer was that simple then someone else would've figured it out already. This incorrect inadequacy analysis is from a July 1603 letter that Kepler wrote to David Fabricius: "I lack only a knowledge of the geometric generation of the oval or face-shaped curve. [...] If the figure were a perfect ellipse, then Archimedes and Apollonius would be enough."

I could make some guesses about why it didn't happen sooner (e.g. the fact that it happened right after Brahe collected his data suggests that poor data quality was a hindrance), but it feels pretty speculative. I wonder if there have been / could be more quantitative analyses of this question, e.g. do we have the data sets that the ancient Greeks used to fit their models, and can we see how well ellipses fit those data sets?

And given the messiness of Copernicus’ system, there simply wasn’t enough evidence to conclusively decide in favour of heliocentrism at least until Kepler’s ellipses - which were only discovered because Kepler had already devoted his life to the hypothesis.

What about Tycho Brahe's comets? Didn't they smash the idea of the crystal spheres in which were embedded the celestials?

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EDIT: Also, the link to Jacob Lagerros' essay lead me to a pay which just said "coming soon".