Why is Islam only acceptable in front of Allah?

The reason is because Allah wants us not to be fooled by Satan, or his attractions, just like the way we are involved in materialistic lifestyles.

All religions are based upon materials, decorations, very colorful churches, temples etc. And when people visit there, they fall in a complex of wearing expensive clothing, and to make sure they look very decorative. In Churches, and temples worlly things are involved, such as pianos, statues, pictures, singing etc. 

If any material satisfies our souls, then that means that religion is fake, because this means we are not worshipping to the creator, and to the creations, also our souls are not a material, so between Allah and the soul, there absolutely shouldn't be any materials involved, and the soul should be directly linked with Allah in worship, without any kind of support, and this is the true religion from the creator.

And the books of people started to worship statues, and materials, this means that they are changing the real message. Stubbornness, and jealousy has made them more further away from Allah. And whoever does not believe in Allah, or his verses, then naturally everyone will have to die, and answer for there deeds, and there punishments accordingly.

Truly, the religion with Allâh is Islâm. Those who were given the Scripture (Jews and Christians) did not differ except, out of mutual jealousy, after knowledge had come to them. And whoever disbelieves in the Ayât (proofs, evidences, verses, signs, revelations, etc.) of Allâh, then surely, Allâh is Swift in calling to account. (3:19)