The easy answer is captivity. In the wild a reptile will rarely encounter its own faeces again so reinfection is unlikely. I captivity the chance of meeting your own faeces again is quite high so reinfection is common. This leads to superinfection i.e. infection at levels much higher than the host or parasite ever intended. I generally recommend to my clients that zero coccidia is the only safe level in a captive situation. Outbreaks generally occur when large numbers of animals are housed together, are stressed (food competition, aggression, transport) and hygeine is not perfect (but it does not have to occur is squalor either).