1) The same reason we go to church on any other Sunday: to worship and honor the God who created us and loved us so much he gave his only Son.
For our family, the strongest reason to go to church at any time is wrapped up in the same reason we are Anglicans in the Catholic tradition: to receive the sacrament of the Body and Blood of Christ. If you truly believe that Jesus instituted Holy Communion for his Church, as a means of transforming grace, why would you not want to receive as often as possible?
But let's say you're in the broader Protestant/Evangelical tradition, where "church" means to come together to sing a few songs and hear the preacher preach. Do you not do so because you believe that, in a special way, God is there? Why would that change because you don’t have to clock in the next day? If worship is just one duty among others, such as work and dental appointments, maybe your worship needs some more, uh, worship.
2) We're better than you.
In fact, just the opposite is true. We are keenly aware of our faults and failures, our sins and offenses. We need more and more of God in order to be made more and more like him, and we have a long way to go. So any chance we get to get molded, off we go!
3) Our parents took us when we were growing up.
Both Rebecca and I grew up in Evangelical households that took church seriously. If we were on vacation, we picked a church and there we went. On a purely pragmatic level, it's usually a good experience for kids to see how other Christians worship God. But more importantly, we learned growing up that a sine qua non of following Jesus is going to church.
Your turn: what are some other reasons you attend church while on vacation?