Why are there not as many Christians now than there were in the Bible?
In reality, it only seems that way. Roughly 1/3 of the people identify themselves as Christians today. That works out to around 2 billion or so people. There is no way for anyone but God to really know for sure how many of these have really accepted Christ as Savior and how many are just showing up at church because it is what they think they should do. Hopefully, the numbers are really that high and growing.
But the demographics are not necessarily looking good. If you go to public schools, they seem pretty rough - to hear my kids talk about it at least. It seems harder to be a Christian because the bad in the world is no longer hidden as securely behind locked doors as it once was and this shows up in all age groups.
But the numbers are still on our side, and there are certainly many more Christians today compared to Bible times because there are simply many more people.
In addition, keep in mind that the Bible is primarily about the Jewish people and the Christians. Neither group has done perfectly throughout history as we are all human, and sometimes people do not follow Christ but still call themselves Christian. If these are people of power, they can lead people astray. Even so, the Bible makes it look like things were better than they really were in history.
Most people don't read the Old Testament enough. If you read it, you will get a glimmer of just how bad the Canaanites were with their idolatrous religions. If you read other historic accounts of the Roman Empire or the Greek Empire or others that made up Daniel's statue, you would see just how nasty and vile and corrupt they were and things wouldn't look quite as bleak today.