Healing is part of the 'atonement', Christ's redemptive work on the cross.
Numerous supporting scriptures also attest. He heals all our diseases, Psalms 103:3. By His stripes we were healed, Matthew 18:7 & Isaiah 53. He bore our illnesses, 2 Peter 2:24. None of the diseases of Egypt, Deut 28:60.
The issue of healing is a rift in the body. While some claim all miraculous sign gifts have ceased, others claim to have witnessed and even participated in them. The difference between dialectical argument and experience gets lost in the shuffle, and despite the availability of videos, testimonies, and real life encounters, people are not obliged to believe (by them).
While some claim a partial and some a complete cessation of the miracles and healings demonstrated in the life of Jesus through the written Gospels, other testimonies throughout history have demonstrated supernatural endeavors (such as the Catholic church, which only beatifies those who have some level of miracles). Even today, while some would doubt, some ministries claim to have upwards of 500 people literally raised from the dead in the past few decades, in the midst of their evangelistic and missions work to the poor of Southern Africa and parts of Mexico.
Indeed, healing tents only several decades ago dotted America in the 1950s, as a wave of healing evangelists went across the nation. Excess and moral failure ultimately caused it's collapse, bringing reproach in general to the subject in the current day.
Theologically, however, healing is one of the hallmarks of Christianity, at lest in some circles. Christ came and demonstrated it, taught faith, and said anythibg was possible if we believed. Men such as Kenneth Hagin took Him at His Word, and is reported to have witnessed many uncounted miracles in His ministry.
As to why some might not get healed, therr are a variety of reasons, other than simply the catch phrase, "Uou don't have enough faith, brother". Indeed, Paul mentioned a few practical reasons for sickness and early death in the Corinthian church in his first epistle.
But, regardlesd, while some would disagree with healing being normative for today, others wholeheartedly day it is. And, as for who gets healed and who does not, usually, the best answer is, thank God for what He is doing.