Well, this is from my own investigation but there are three strong reasons(people could add their own as well):
Money. NATO shills, through US funding, advertise big salaries to ex paramilitary soldiers, ex military, and ex police men. Those salaries are simply hire to whatever Colombian drug traffickers or even the gov’t offer so these people take the bait. Example down below with 2.8k USD as salary for mercenary work.
Fascist sympathizers(example in Spoiler). There is just a lot of fascists that have been trained by the USA from years of struggle with the guerrillas. Also, the US have plenty of bases in Colombia which will leave you wonder: Why does Colombia have so much land used to farm drugs? The answer is very similar to Afghanistan’s US occupation years(good book on this is Seth Harp’s The Fort Bragg Cartel).
spoiler
Strong influence from Ukrainian/American to form sympathy for fascists and dehumanization of Russians. This one is in great part due to US corporate media.
Well, this is from my own investigation but there are three strong reasons(people could add their own as well):
spoiler