Because, unlike many Christians, they were raised to feel real empathy for other people, and not to think of others as tools for obeying God. When they saw a person in need, they helped them because they felt true compassion for them, and their sincerity was clear. They did not offer services to people just because they felt God expected it of them.
When a person uses another in order to obey a third entity's orders (God's) that second person, and their real needs as a human being, get lost in the shuffle. The first person believes they are being loving, but they are often only willing to love on their own terms, and it tends to have strings attached (like, getting the other person to convert, or the other person must show inordinate thanks and praise for the oh-so-godly person). The Wiccans I've known have just *cared.*
No, sorry. You didn't think hard enough.
What I am about to say is shameful to humanity, but true nonetheless. Most people - Christian, Jewish, Hindu, whatever - are born into their faith, and identify themselves as belonging to that faith, but could care less about it. They are far more concerned with themselves and their petty lives. They just say they are 'x' faith because they were born into it.
Now wiccans are generally people who were not born wiccan but found it on a spiritual quest, the bigger questions are important to them. They care about their place in this world and they care about others. They are spiritual seekers. They are most likely very good and very unselfish people. And very moral.
Now again, most Christians are just Christians because they were born that way, they've never given it any real thought. They go to church on Sunday because their neighbors do and its expected. They mentally sleep during the service.
But. And a big but. There are spiritual seekers in Christianity too (and other religions as well). People who have sought and found inspiritation in Christ's message. These people would be every bit as good a person as any wiccan. They have a hunger to understand the grand design and an intuitive knowledge that a large part of it is sacrifice for your fellow man.
Now, if Wicca was what most people were born as, and few were Christians you would see the reverse. Most identifying as wiccan would be selfish, base humans (because that's what most people are), and the few who sought out spirituality and found Christianity would be shining examples of how humanity should be.
