Why do so many countries seem to favour Christianity over other religions even when they say they are multicultural and include every religion?

Go to a mosque or a synagogue and ask for food money

The various religions prosper in areas where they're taught from infancy. If you were born in Tikrit you'd probably be a Sunni Moslem, if you were born in Tel Aviv you'd probably be Jewish, if in Latin America you'd be a Catholic, in the US Bible  Belt you'd be a Baptist, and in Scandinavia you'd be Lutheran (except for a small contingent in Iceland where a temple to Thor and the Norse gods has recently been built).

When the majority of a population shares a faith it becomes, through sheer weight of numbers, the predominant religion. It's not terribly important because each of them us happy in the conviction that
theirs is the "true" religion.

Although Christianity is naturally preferred  in the so-called Christian countries that doesn't exclude others from worshipping as they choose -- except, of course, when zealots and bigots try to impose their own beliefs on others. Most of the other major religions are equally tolerant except for Islam as it is taught in the Islamic theocracies. Apart from their hatred of the Jews they have also persecuted other minority groups, including the Zartushties whom they hounded out of Iran to their present home in northern India.

Compared to them, Christianity can be proud of its stance -- the aforementioned bigots and zealots notwithstanding.

It's a curious thing. In the past 200 years, Christianity has become (with a few sectarian exceptions), the most tolerant of all the religions. Where Christians are not persecuted by those of other faiths, they just manage to "rub along" with everyone. I'm not a Christian, but I do recognise the attractiveness of "doing the decent thing" that goes along with the religion. With the possible exception of some forms of buddhism there is probably no other belief system that works that way. We pride ourselves in the UK for our ability to absorb those of different faiths and cultures. It is the solid Christian grounding in which the establishment is based that has allowed this.

What's not to like?

And as I said - I'm not a Christian...

Because the people who are Christian end up having children who grow up Christian (most of them, not all).

True Christianity is not dictated by nationality. The bible says at Acts 10:34 ".....God is not partial, but in every nation the man who fears him and does right is acceptable to him". The bible warned that there would be an apostate form of true Christianity that would spring from what was formed by Jesus  2Tim 4:3,4. This watered down version of Christianity has been very misleading and has served the purpose of the one's starting it, control and money. That being said, a sincere and humble person can still find what Jesus left his followers to follow if he sticks with the bible and not self appointed leaders that claim that God talked with them and you must listen to them rather than the bible.

Christianity has also been evangelized with much more fervor than many other religions. With some form of "Christian" consistently going into areas with differing beliefs and offering assistance, and sometimes using violence, Christianity has  become integrated into a large number of societies throughout the world. Historically "Christianity" has brought money, assistance, stability, and intimidation. At a certain point it just becomes easier to accept it.

It's a specific type of denomination Catholicism,  which means universal. The Catholic church contains all pagan worship. If you've read the history of the Catholic church then you will understand, the Pope laid claim on many lands under the guise of saving souls. The African slaves found Catholicism easy to incorporate into Haitiun Voodu because the Saints are assimilated with the Loa (spirits/anima). As you are aware the Catholic church worships Mary, aka Goddess Mari.

https://en.m.wikipedia.org/wiki/Mari_(goddess)

https://m.youtube.com/watch?v=IXFfKmCbq0s

(See part 2)

The bent cross from Odin.

www.eaec.org/cults/romancatholic.htm

All churches are probably pagan. Church comes from Circe and Goddess Circe or Psyche known for magic and manipulating men's minds, the steeple is still the phallic symbol of Osiris and the Dome is Ishtar or Isis. The Circe - circus, circle i.e. witchcraft.  Wicked comes from Old English wicca (witch) -ed.

They have the most holidays?