Ethanol is 2 carbon atoms, 6 hydrogen atoms and one oxygen atom. THC is 21 carbon atoms, 30 hydrogen atoms, and 2 oxygen atoms, arranged in a much more complex molecular structure.
Because there is so little heat in space, it is extremely rare for complex molecules to form - there's not enough available energy to sustain most chemical reactions. Most of the matter in space is in elemental form, or very simple molecules like water. Even the presence of large amounts of alcohol was baffling to scientists at first, and there are various theories involving concepts I can't really even pretend to understand like quantum tunneling to explain how it might form at such extremely low temperatures. Bigger molecules are exponentially less likely to be able to form naturally in space. Finding alcohol in space was quite a surprise. Finding something as complex as THC would be absolutely mind-blowing.
