There are IT firms which prefer Linux operating systems but at the same time there are softwares which can run only in Windows Operating system hence companies have to install windows operating system.
If you look at the advantages of Linux over Windows there are several pro like :
- Open Source and free ,while MS products are costly
- Security aspects are much strong in linux
- No hardware incompatibility
If you look at stackexchange Survey on Operating System Used you can see Windows top the list.