MATIC is the native token of Matic network and it has a contract address - 0x0000000000000000000000000000000000001010 on the Matic chain. It is also used to pay gas. Mapping the MATIC token on the PoS bridge will lead to MATIC having an additional contract address on the MATIC chain. This will collide with the existing contract address as this new token address can not be used to pay for gas and will have to remain as a normal ERC20 token on the matic chain. Hence, to avoid this confusion, it was decided to retain MATIC only on Plasma.