One possibility is that this passage is partially about denial, and that "coming to the light" does not refer to salvation, but to the process of abandoning our denial which is part of our journey to salvation - after all, "whoever does evil does not come to the light, that his deeds not be exposed". Salvation does not expose our evil deeds - but there is a step before salvation, and that involves coming into the light. I think that the opposite of "coming into the light" may be denial, and I think that 1 John says some important things about denial versus truth. So I think that this one of the big reasons people do not come to God is that they are in denial and do not want to be exposed. It is interesting that Jesus is saying this to Nicodemus, who came to him by night so that nobody would see him talking to Jesus.
>and if doing "the truth" refers to the practice of righteousness
I suspect that "doing the truth" here may mean quite literally practicing truthfulness.
It may well be that one of the big reasons people do not come to God is that they are afraid to look at themselves, and that having the courage to see ourselves clearly is an important part of coming to God.
Jonathan