It is a matter of end points for fusion reactions and which elements are gases vs which are solids. Hydrogen is the start point, Helium is the primary product of hydrogen fusion. After that, the carbon cycle produces Carbon, Nitrogen, and Oxygen.

Free hydrogen is the most abundant element but as a gas, you have to get a lot of it together before gravitational contraction will form it into a sphere. If it is large enough, hydrogen fusion begins and produces Helium. Apologies to the mods, but a link is the best explanation of this process. http://hyperphysics.phy-astr.gsu.edu/hbase/astro/procyc.html#c1 Pure Helium is unlikely to form a planet because - as a gas - there is no reaction to cause it to coalesce in anything near a pure form. Similar limits affect oxygen, it is a gas and there is nothing to trigger it to coalesce in pure form.

Carbon is formed in a secondary reaction during hydrogen fusion, then becomes the key ingredient in the carbon catalyzed fusion cycle. Because the reaction is "catalyzed", Carbon tends to accumulate in these large stars. The Carbon cycle starts with a Carbon nucleus, but because the reaction is "catalyzed", one of the end products is a Carbon nucleus. Carbon will therefore accumulate in these stars. Large stars tend to go supernova and in the process shed huge amounts of carbon, nitrogen, and oxygen - primary products of the carbon cycle - first in their stellar wind, and later in the supernova. This free carbon becomes part of the interstellar medium as fine dust. Carbon is therefore the second most likely element to form a planet because it is abundant in the interstellar medium and stable enough to readily clump into granules. The granules are able to clump until a planet is formed. http://hyperphysics.phy-astr.gsu.edu/hbase/astro/carbcyc.html#c1 and https://en.wikipedia.org/wiki/Carbon_planet and http://www.scientificamerican.com/article/carbon-planets-turn-earths-chemistry-on-its-head/