I would imagine that to be born in the time of the Buddha would be very good Kamma. And that people would have to have that good Kamma in order to get in contact with such a person. So it appears to me (and I don't know this to be true), that people born in the time of the Buddha had enough Parami development that enabled them to be born at that time. So they perhaps had better potential for enlightenment than us say, who are born at a time when we don't get the opportunity to meet such people.
As far as the teachings go, from what I have read, it appears that in order for people to get enlightened they have to receive instructions and methods given by enlightened people. So part of the decline of enlightened people may have to do with perhaps not so many people becoming enlightened and that keeps on decreasing. Because, although we have the teachings of the Four Noble Truths, do we actually have the appropriate conditions? Spiritual people around who can help us?
And the Seven Suitable Conditions may not be available - the most important being suitable friends and suitable conditions. Especially in the modern world where materialism is much more predominant and people's lives are so busy. With materialism predominant, what is important and what is not doesn't seem so clear. People's lives are much more complicated and getting the conditions suitable for practice may not come as easily as they did back then when society was not so developed. This is how I understand it.