...and why should the Saxons and Danes care more about the trade with the Arabs than their neighbours the Franks.
Christianity: the only rules forced on the Pagans was zero tolerance for other faiths and no human sacrifices.
Islam: Zero tolerance for other faiths, no human sacrifices, no eating pigs, no drinking alcohol, no using the blood of slaugthered animals and cut something of your dick.
Beside that Christianity had adapted to European culture for centuries, while Islam hadn't.
1) Probably for the same reasons as the Italians: the Muslims have better stuff/exotic goods, and are right there. With much of Europe under Muslim rule, the Saxons and Danes COULD convert. They wouldn't necessarily, but they could.
2) Christianity: Encouraged chastity, overturned almost all the pagan social institutions (excluding the ones it adapted/absorbed), suppressed pagan philosophies, required a complete overhaul in worldview. It's less of a leap from Christianity to Islam than Paganism to Christianity, honestly.
3) Did I say it would happen instantly? I figured the fact that it would be a very gradual shift, if it happened at all, was obvious. And of course Islam would be adapted to European culture in the long process, just as it was in China, India, and Malaya.