In my opinionpeople tend to ignore God because the people that surrounds them influences their attitude and beliefs.
Some people ignore Him because they choose to be "free" and do whatever they want.

Some are tired to go to church, tired to hear and understand His words, tired to spread gospel, tired to do God's will, etc.

Some even think that knowing God is just a waste of time and effort because some think that He's not real.

Some are Christians, but they ignore Him because some ignores and forget that they have a God.

I guess those are my opinions. :) 
God bless you ^_^

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Maybe because they don't believe in God, or maybe people around them that didn't believe in God influences them.