there is bitcoin which is the trusted decentralized currency that people know of and have been using for years and has the majority support. then one day last year someone decided to copy it and create their own version of it and try to double the amount of coins they have so that they can earn some free money. so they created bitcoin-cash.
this year someone else wanted to do the same so they copied the copy coin and created bitcoin-sv out of it and made a lot of money in one day.
Why people get BCH coin for mining new block in Bitcoin SV?
they do not. you are making a mistake.
if you find block of coin X you will get the reward for coin X. if you find block of coin Y then you get the reward for coin Y.