It is 100, because that's what Bitcoin's creator set it to. It's part of the system's consensus rules now, and cannot easily be changed. It also doesn't really hurt.
In retrospect, it is likely too high. The purpose of the maturity is making sure that when a deep reorganization takes places, no transactions that directly or indirectly spend coinbase outputs exist, as those will be invalidated. Given the fact that reorganizations that are more than a few blocks are extremely rare, 100 is likely overkill. Then again, it's a good thing the value is higher than what is ever needed, and again, it doesn't really hurt anyone.
What happen if I will decrease my coinbase maturity to 50? It will affect different parameters of the bitcoin network (block creation time, target timespan, targer spacing, etc.)?
The network will simply reject your transaction, or at the very least the transaction will not confirm until the coinbase outputs it is spending are 100 old.
The Bitcoin Core wallet used to actually enforce a coinbase maturity of 120 before permitting spending. Note that this was a local wallet policy, distinct from the consensus rule that requires 100. This policy was removed, as 100 turned out to be more than enough.