Let's turn
siwr's write-up around, shall we?
Perhaps the reason that some Democrats will cross party lines frequently is because of their own party's closed-mindedness. The point has been made here on E2 before, but it bears restating: some Democrats (particularly, as mentioned, Reagan Democrats, many of whom were Southern Democrats back in that day) may be open-minded, but their party itself is notoriously intolerant of dissent.
At the 2000 Republican National Convention, Gen. Colin Powell delivered a keynote that had all the hallmarks of a public spanking, particularly as related to affirmative action and the Republican party's opposition to it. However, he was received enthusiastically despite this disagreement with a key point of his party's platform. Conversely, in 1992 Pennsylvania governor Bob Casey was denied any opportunity to speak at the Democratic National Convention in New York specifically because of his opposition to abortion.
Perhaps the reason most Republicans have a high degree of loyalty is because of the diversity of opinions represented under the Republican banner. If individual Democrats don't agree with the party line, they can either change their minds or shut up. Individual Republicans are free to speak their mind, and will even get support from their party to do so.