The simple reason - because one speaks Dutch in the Netherlands. God is Dutch (and English), Allah is Arabic. Arabic is a beautiful language, so why not just switch over completely?
---
The Bishop of Breda, Tiny Muskens, thinks "Allah" is a beautiful word for God and suggests it from now on. He is aware that using the name "Allah" would be very emotional and therefore he says it must be thought about, prepared and weighed in advance.
However, he is convinced it will happen, whether it be over a century or two. Muskens had worked in Indonesia for eight years and had prayed there to Allah, saying he had called God "Allah" during mass.
Source: Trouw (Dutch)