I have seen almost all operating systems and large sized files are in ISO format. why they choosen that format instead using any other formats like zip, rar, etc.
Answer
Ordinary large files, in general, could be distributed as .zip archives (any format would do, but Zip has the most support across systems). There is a big difference when you're distributing operating system installation files, however.
The formats you have listed are not disk image formats: they only contain files but none of the surrounding filesystem structure, e.g. no boot sector, no partition types. They can be understood by programs on the already-running OS, but not by the firmware to start an OS from.
To create a bootable disk, you would have to convert the .zip archive into some other format that's recognizable by the computer's firmware – e.g. BIOS-bootable disks must have a boot sector, or a partition table with specific partition types for UEFI, or an ISO9660 filesystem for CDs/DVDs.
If you were using UEFI, it might be enough to simply reformat the disk using FAT32, and extract the files from a .zip archive onto that disk. But the user would have to be careful to pick the correct filesystem type, to not accidentally move or delete some files, and so on.
And that's not enough for BIOS systems, and not quite that easy for bootable CDs either – you would need to distribute at least two files; the contents themselves plus the boot sector; and trust the user to correctly write both.
So instead of making every user do this work separately (possibly with differing results), the OS distributors do the conversion themselves, and provide the final image for download, ready to be written 1:1 to a disk.
(The files are called ".ISO" because they originally contained a 1:1 image of a compact disc, in the ISO 9660 format. The naming convention continues even for UDF or hybrid CD/HDD images.)
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