Western nations have a tendency to adhere to scientific, medical and pharmaceutical principles, especially in relation to psychiatry. Developing nations may simply be unaware that schizophrenia exists, let alone to recognise the symptoms outlined within the DSM-5. Moreover, the DSM was devised by psychiatrists in western countries, so they have their own diagnostic system with the disorder included, which inevitably leads to a higher frequency of the illness being identified.
It's also worth noting that developing nations may not consider classic symptoms of schizophrenia to be an 'illness' as western cultures may deem it to be. In the UK, it could be seen as delusional to have a vision of God, but a person in a highly religious country might genuinely consider that to be evidence of a deity's existence. The cultural interpretation of certain perceptual, psychological and behavioural events significantly influences whether it's considered pathological or not. In the west, we have a tendency to diagnose, medicalise and pathologise experiences which other people may believe are spiritual or religious. Therefore, we diagnose these people as having illnesses and use the standard DSM-5 or ICD in order to do so.