The Watchtower Society (WTS) does not have the monopoly on religious control over many adherents' minds and lives. There are countless "mainline" Protestant groups, mostly independent "non-denominational" churches, whose dynamic is just as cult-like as the WTS. I know plenty of such folks who have belonged to such churches -- Trinity-believing, "spirit-filled," Bible toting, evangelistic, moralistic churches -- and whose lives were turned upside down due to the persuasive power of indoctrination programs and the control of its leaders. I personally know a couple who were intensely devoted to such a church and raised their kids in this church, only later to be forced to leave because they began questioning the leadership's methods of control. This couple, who are dear friends, remain scarred to this day, even after many years. And they continue to reflect on how their experience deeply shook their own faith and destroyed the faith of their children, all of whom are now vehement atheists, citing their experience growing up in this church as the cause of their atheism. (The group they had been part of was a "People of Destiny International" church, which later became Covenant Life Church, which then became Sovereign Grace Ministries.)
Such experiences in Christendom are far more common than people realize. In fact, given the general definition of "cult," I would argue that most churches would qualify as cults. Specific indoctrination programs, membership requirements, a certain level of necessarily expressed obedience to a leader (such as a bishop, or a pastor, or a body of church leaders, elders, etc.), manipulation of financial obligation, and even the imposition of disciplinary conditions.
Now, of course there are varying degrees of intensity as to how the above factors are applied in any given church or group. The Watchtower Society has disfellowshipping, the Amish have shunning, the Roman Catholics have refusal of sacraments as one means of excommunicating a parishoner (which to Roman Catholics is as impacting as disfellowshipping is to JWs), many Protestant "non-denominational" groups have forms of acute ostracism akin to shunning, and so on. But what each such group has in common is the drive to maintain core authority, all of which is grounded in some kind of key doctrine and/or marketing program (read: "evangelism") particular to each group's leadership (e.g., the "prosperity gospel," probably the biggest money-making scheme seen in Christendom in the past twenty years and which has been a huge stumbling block to countless thousands, if not millions worldwide). In the early 2000's I ran into a number of folks from the International Churches of Christ, which by then had been highlighted as being not only the fastest growing church at the time in the U.S. (a short-lived distinction), but also a cult. Its members (at least the many I encountered) were fervent and aggressive. You think the WTS is bad? The ICOC outran it by miles. As I recall it burned out some years ago.
Again, the WTS does not have a monopoly on religious manipulation and control of adherents, or on false doctrine. I would even go so far as to say that, compared to the countless harmful groups that exist within Christendom and which are in large supply, the WTS is remarkably tame.
And so this goes to the question of the originating post: "why haven't the major religions of Christendom exposed JWs as the true apostates"? I suspect it is very likely due to the fact that there are far more dangerous groups within Christendom that the WTS cannot even hold a candle to, and which are therefore of greater concern to the major religions of Christendom.
