Seeing Miracles Does Not Result in Belief
There is an invalid assumption in the question, specifically that the viewing of miracles makes it easier to believe in God. If this were so, then all of the people who saw firsthand the miracles that Jesus performed would have believed in Him. That, however, was not the case at all. In reality, many of those who saw the miracles responded by hating Him, attributing His miracles to demonic powers, falsely accusing Him of blasphemy, and calling for His death.
The Apostle Thomas at first refused to believe in the resurrection of Jesus without seeing Him for himself. However, he was not commended for that demand. Rather, those who believe without seeing were commended. So, indeed, believing without seeing is most certainly possible, and even the preferred method.
Also, when Jesus told the story of the rich man and Lazarus, the rich man asked that Lazarus be risen from the dead to go back to the rich man's family and warn them. However, the rich man was informed that even if someone were to rise from the dead, people would still not believe.
Many Believed Without Seeing Miracles
Even in the first century, immediately after the resurrection of Jesus, thousands upon thousands of people became believers without seeing any miracles. It is true that there were still some miracles being performed, but not for every individual to see.
The question, then, is what it was that caused so many people to believe if they saw no miracles. The answer is that they saw the transformed lives of the apostles. Even the religious leaders of the day took note when Peter and John, who were unschooled, ordinary men, stood before them with an astonishing courage and proclaimed the resurrection of Jesus and salvation in His name. (Acts 4:13)
Foxe's Book of Martyrs records how many people believed when they saw the faith of others in the face of persecution and death for their belief in Jesus. They faced death so peacefully and they so steadfastly refused to denounce their faith that others could only conclude that what these people believed had obviously had supernaturally transformed them.
Others were persuaded based on the evidence of the fulfilled prophecies. Stephen reasoned with people, proving from the Scriptures that Jesus was, in fact, the Christ.
The Work of the Spirit
An important thing to note is that belief does not come from merely physical evidence. The Spirit of God testifies to the hearts of men of the truth of the Gospel. Christians understand that mankind is not purely physical, but also spiritual--that is, we have a spirit as well as a body (and a soul as well). God testifies to our spirits directly.
The Complete Picture
So, people believed in Jesus in the first century based on 1) the transformed lives of others who were Christians, 2) the evidence of the fulfilled prophecies, 3) the witness of God directly to their spirits.
Some also saw miracles, of course, but that again did not automatically result in faith.
Today, people believe in Jesus for the same reasons. Some today do see miracles, but those are often seen by those who are already Christians.
The issue really is not an issue of having enough evidence to believe. I would contend that there is enough evidence to convince someone, beyond a reasonable doubt, that there is a God and that Jesus is the Son of God who became a man, died, and rose from the dead. It is not an issue of evidence, then, but of choice. As the saying goes, "A man convinced against his will is of the same opinion still."