I'm reading https://en.bitcoin.it/wiki/Difficulty
And I don't get why max target is not
(2^256 - 1) which is
If the maximum target was 2256-1, every candidate block would be a valid block.
As even the CPU miner in Bitcoin's first software release was capable of doing 100 kH/s or more, this would have led to a very rough start of the chain, with 1000s of blocks produced per second until the difficulty adjusted.
Because of that reason, the maximum target was probably set to a level that would guarantee not too frequent blocks after genesis.