1. Because it can be derived directly from a section of a cone in three easy steps, as demonstrated in section 6 of A E L Davis, Some plane geometry from a cone: the focal distance of an ellipse at a glance, Mathematical Gazette, forthcoming July 2007.
2. Most accessibly in A E L Davis, Kepler's unintentional ellipse - a celestial detective story, Mathematical Gazette 82, no.493 (1998), p.42.
3. Fortunately, the incompatibility continued to worry Kepler, and motivated him eventually to work out a more effective approach to the mathematics of an elliptic orbit: see Sect.6 of A E L Davis, The Mathematics of the Area Law: Kepler's successful proof in "Epitome Astronomiae Copernicanae" (1621) Archive for History of Exact Sciences 57(5), 355-393

Detailed references at this link

MacTutor History of Mathematics
[http://www-history.mcs.st-andrews.ac.uk/HistTopics/References/Keplers_laws.html]