Bitcoin is just like any other secure means of investment,it allows you to save funds without any interference from any party or government.You have full control over your funds and can basically decide whatever you want to happen,that's an important reason that makes bitcoin systems safest.
And I think one of its characteristic is its anonymity that can't trace who are the owners of bitcoin addresses and the untraceable transactions it can made. So maybe this is all the reasons why bitcoin is the safest system.