Because this world is NOT heaven – this world is ruled by the Devil and still under his dominion. In this world, when God does heal – which he often does… God does not heal for the sake of healing as we so often wish – as he will do when we are all resurrected and given perfect glorified bodies as we once had in Eden.
No – he heals in this world for another reason – to display his light in a dark world! To show that HE is still the master and is able to contravene the laws of nature! To show He is still on the throne and has our backs – even in a world handed over to the Prince of Darkness!
When God doesn’t heal – NEVER curse God. That is Satan’s main aim for you – to curse God and blaspheme the Holy Spirit! Just believe in His goodness and His ultimate plan for our ultimate healing and happiness and joy in the soon-coming renewed heaven and earth!