Okay. It seems like people weren't satisfied with my attempts to give a simple and concise explanation, so I guess I have to give a longer explanation.
The biggest blow to Christianity happened when Darwin published his theory of evolution in the Origin of Species. It directly disproved the concept of intelligent design and gave an answer to the question of "How can there be so many diverse and complex forms of life?" that didn't rely on the existence of God. This question was a big deal back then because prior to evolution, it seemed like such an absurd and impossible fact of existence that "an Omnipotent Deity did it" was seen as the only viable explanation. The church knew the threat Evolution posed, which was why they did everything they could to suppress it, just like they tried with the printing press way back when (If only the church knew how to copy oil company tactics).
The second biggest blow to Christianity was the information age/Internet. The church has always relied on the ignorance of the populace to draw its power, which was why they tried to suppress the printing press (giving absurd reasons for why the printing press was 'satanic'). Now knowledge was at the fingertips of almost anyone and subsequently we've been seeing a decline in Religion in the West.
As part of my previous oversimplification, I said that a reasonable and scientifically minded person would inevitably discover evidence that contradicted the existence of God. That's not really true. Due to the fact that God is in many ways outside of science, you won't find any evidence that directly refutes his existence. What you will find though is evidence against a given church's Dogma or interpretation of God. Young Earth creationism is contradicted by the fossil record, the theory of evolution, and the fact that scientists know the earth is actually quite old. We know God isn't an all-loving interventionist because he does not do enough to help with the problem of evil. You've also got circumstantial evidence against the church being correct due to knowledge of its shady history.
That's not the most important thing though. As science has progressed, we've relied less and less on religion to answer our common everyday questions. We now know where the rain comes from, why people get sick, the mundane origins for awe-inspiring events like earthquakes and hurricanes, and we even figured out most of the stuff involving the origin of life, our planet, and the universe. God may still be real but the truth is that we don't need him as much as we used to. And as a result, more and more people are choosing to go without him.
In regards to your most recent statement Iduno, I apologize for using absolutist language. Again, I had oversimplified. The moderate branches have not died out, but they are dying. It's a fact that churches have been losing followers and many of them have shut down as a result. Buildings that have lasted centuries are now being demolished to build condominiums. This has been happening since before the Pandemic. The fundamentalist churches have been more resilient to this trend for the reasons that I outlined previously.
Islam on the other hand is actually growing last time I checked. This growth is happening in the middle east though, but that's due to a combination of political instability and the efforts of theocratic governments. Over there, there are more important things to worry about than the question of whether God exists or not. Plus, the people really need the hope religion provides.
TLDR: For most of human history, there wasn't much evidence against God, but modern technology has accelerated the spread of this evidence once it became available. Now Christianity is dying, but fundies are doing it more slowly. Currently much of the Middle-East is far from being a bastion of free-thought and discovery.