(Thanks @Skyge as always for being awesome 
)
State variables being private is part of the OpenZeppelin Contracts style guidelines:
https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/GUIDELINES.md#solidity-code
The explanation for this can be found in the Release notes:
_From: https://github.com/OpenZeppelin/openzeppelin-contracts/releases/tag/v2.0.0_
All state variables are now
private, which means that derived contracts cannot access them directly, but have to use getters. This is to increase encapsulation, to be able to reason better about the code.