Software/music/movie pirates usually fall into very few categories:
* never willing to pay for it
* willing to pay, not willing to put up with current and known barriers
The first category should never be assumed to be a lost sale, however the second could be assumed a lost sale.
* Cheapos. Won't pay for it unless it is the only way to get something that they like. (it's hard to find pirated copies, the risk (legal, malware etc) is high etc)
The simple explanation would be that Android users pirate because piracy is trivial. Other more nuanced explanations may be that Android users are cheapskates, or hold strong beliefs against paying for software. I'd like to be charitable and assume it's just because downloading APKs is easy.