I see two reasons for this.
First is simple peer pressure. Within the evangelical Christian context, there is a very strong "encouragement", if you will, to identify with the far-right wing of the Republican party. People have left churches I've been in because they were "too Republican" and conservative pastors and congregants routinely praise GOP politicians and criticize Democrats by name, from the pulpit and the head of Sunday school classes.
The clear message given is: "If you are not a Republican, you are not a Christian". You can see verification of this right here on Christianforums where you see posts making ridiculous claims like "Obama is the anti-Christ" and "John McCain is a liberal." (Interestingly they only criticize GOP politicians when they are not far enough to the ultra-right)
Placed in this environment, many will want to fit in by adopting the positions of their peers. Positions in church leadership and the congregational social pecking-order depend upon it, so people go along with the group-think to avoid ostracization.
Of course, a similar dynamic exists amongst more moderate Christians, but they aren't nearly as strident or punitive, in my experience.
Secondly, one may adopt the group opinion for the purpose of coalition building.
For example, I may be interested in working to correct the many abuses of the American corporate culture. I may quickly find that many who share similar interests also are interested in, say, reproductive rights. Now, I may have little interest in the women's rights movement, but I may adopt some of their opinions as a quid pro quo because these same people are also helping me in a cause I care about and I don't want to jeopardize that cause by being contrary to their opinions.
