Indeed it would often be difficult when religions is involved, and it always ends up in meaningless fights when no side would ‘win’ anyway. Best not to discuss that (Atheist here, so no preference to any religion)
With football itself, in general, it is most popular in Europe and South America, as well as some Asian countries like Japan and Korea which have festival activities with some of the more poplar ‘Western’ holidays (They do mainly celebrate their own main holidays, but would still have adopted the celebratory holiday mode even when most of them don’t even believe in Christianity at all - to them, say Christmas or Easter, is just another kind of holiday mode break with presents or chocolates for others). Obviously there are a huge number of fans all around the world including Muslamic countries, but then it is normal and easy for an event team to look at the ‘Western’ holiday calendar to come up with some headlines and designs, and it could be more receptive to most audience easily. It would have been a lot more difficult for non-Muslims to understand the culture and meaning of Muslim holidays than the Christian ones (A successful headline should be short, very easily understandable and receptive to attract attention quickly)
Nothing discriminatory at all there in not using anything else, in my opinion. Just probably easier and simpler marketing for a majority.