It is because this is the one way and only way for a human to reach a truth. To put it another way, if God is true then to believe what is said in the Bible is the one and only way to reach such a truth. There's no other way round.
The methodology employed here is called human witnessing. It is the same methodology leading to virtually any truth in human history. The nature of history, under most circumstance if not all, is that you have to rely on what have been written down by other humans to reach a truth. You have to deliver your faith, or else there's no truth for you. That is, you are unable to reach a truth unless you put faith in what have been written down by a small bunch of humans. Most likely, the more witnesses (historians) were writing about the same event/figure, it became more credible for humans to believe (multiple account witnessing).
If God is true, He will choose such a methodology to pass a truth as human witnessing, without man's own awareness, is the most fundamental way of conveying a truth. And that His Word must be accurate (inerrant) to a certain extent for humans along the timeline of history to reach the same God. The same accuracy is demanded if humans are to be judged. If the core information provided is not consistent, humans are not to be blamed when subject to God's judgment, as they received no consistent information.
If a God would like humans to reach Him, He needs a book accurately described about Him and His requirement for humans. Such a book will basically not change along time for humans in different times of history to read the same stuff to reach the same God with the same requirement for His judgment. He also needs to authenticate a human authority as a representative of His book, or else everyone may pop up to say that "my version of the book is the genuine one". The Jews were first assigned to be such a representative. When the Jews became unqualified, the authority was shifted to the Catholics then the Protestants. All of them are maintaining the same core (Canon) of the OT, with the Catholics and Protestants sharing the same NT.
By far, the only God who can achieve the above is the Christian God. And martyrdom is used as an enhancement for the multiple account witnessing. By the covenants man is bound to, He won't show up to all humans (or else humans cannot be saved by faith, there's no faith required if He shows up to everyone). He showed up only to His dedicated witnesses to allow them to write down the Bible for other humans to believe to reach the truth.
Isaiah 6:9
Be ever hearing, but never understanding; be ever seeing, but never perceiving.