The Bible promises healing. No equivocation there.
A lot of people pray for healing, and don't get it. No contesting that, either.
What's the disconnect? Why don't we get the healing for which we ask, when we ask for it?
There is a category of religious folk who would say that it's a matter of too little faith. If you pray with a seed of doubt, rather than the perfect faith of the mustard seed, of course you won't get healed.
What they overlook is that we're not seeds. We were created differently, and doubt is part of human nature. If you want proof, look at Jesus' words in the Garden of Gethsemane, and on the Cross. His human side...despaired. Doubted. Felt abandoned.
So that's probably not it. God would not, I think, promise healing if and only if we can become something he didn't design us to be.
There are others who say you need the anointing. And if you don't have it, there are certain preachers who do, and their healing services will soon set you right. You'll be healed, so to speak, on the updraft.
The services are spectacular - limping people don't limp, deaf people can suddenly hear, crutches are thrown away. Unfortunately, most of the healings are very hard to verify. No one with a severed spine steps up from a wheelchair, and severed limbs don't regrow. The adrenaline rush attendant to these occasions is considerable, and can make it easy to believe that the arthritis that was crippling is now gone. But, alas, it usually isn't.
(The argument against the 'verification' is that it's a symbol of doubt, and therefore cancels the healing. Does this sound absurd to you?)
Another argument against emotion-fueled services is that Jesus did a lot of healing, and he did it pretty casually. Spit in the eye, mixed with dirt, that sort of thing.
But healings do happen. People with cancer suddenly 'lose' it, and that's medically verified. Blindness lifts, and even the Mayo Clinic can't say why.
This, perhaps, is thew 'why'. The healing has to fit in with God's overall plan, and we are simply not told what that plan is, concerning ourselves, or anyone else. If we take it on that basis, we can accept the healings that happen, and accept the fact that sometimes the answer is 'no'.
This begs the question, why ask at all? If the healing is part of the plan, won't it happen anyway? We don't know the answer to that, but we do know that some people prefer to be sick. It gets them attention and pity, and relieves them from the requirement to do some kinds of work. It's the kind of choice - like robbing a liquor store - that's certainly against God's plan, but one He accepts if we choose it. We have to accept the consequences, though.
We demand free will, but we have to allow God free will, as well. And His trumps ours.
And after all, Jesus, in the Garden, did say..."Not My will, but Thine be done."