A Barna survey says many of the unchurched avoid church because of past negative experiences in church:
The Barna data indicate that 28% of the adult population has not attended any church activities, including services, in the past six months. That translates to nearly 65 million adults. When their children under the age of 18 who live with them are added to the picture, the number swells to more than 100 million people. [The six months test is Barna's working definition of unchurched.] ... One of the biggest surprises to some people, however, is that a large majority of the nation’s unchurched population is drawn from the sector comprised of people who consider themselves to be Christian. In the United States, 83% of all adults label themselves “Christian.” The percentage is lower among the unchurched, but such self-identified Christians still outnumber those who do not embrace Christianity by a three-to-two margin (61% vs. 39%). ... Based on past studies of those who avoid Christian churches, one of the driving forces behind such behavior is the painful experiences endured within the local church context. In fact, one Barna study among unchurched adults shows that nearly four out of every ten non-churchgoing Americans (37%) said they avoid churches because of negative past experiences in churches or with church people.Read it all.
Barna also reports,
Catholics have traditionally been more consistent in church attendance than Protestants. However, in the mid-nineties that trend reversed course, and Catholics have been more likely than Protestants to earn the unchurched label throughout the past decade. Currently the gap between the two segments of Christians is five percentage points, with a higher percentage among Catholics (25% are unchurched) than Protestants (20% are unchurched).
Within the Protestant community, people who typically attend a mainline church were nearly twice as likely as those who attend non-mainline Protestant congregations to be unchurched (26% versus 16%, respectively). Also, church size was related to attendance patterns: 24% of the people who attend small churches were unchurched, compared to 15% who usually go to a mid-sized congregation, and just 5% of those who affiliate with a large church (defined as attracting 500 or more adults on an average weekend).