Why is the winner of Pow different everytime? I just read about how miners have to change the nonce bit in order to find an hash below a certain result.

Granted each miner process it the same way, incrementally, adding 1 and hashing, adding one and hashing, what prevents the most powerful computer on the network to snatch all rewards?

I feel like beside the moment each miner starts mining a block, there's no room for luck and a lesser powerful machine, to ever add a block.
Because each miner may select the transactions (and the order in which they are included) in each block. Also, they do not all start on the same exact nonce, in fact they are changing more than just the nonce in many cases (e.g. block versions - see ASIC BOOST).

This means that they are not all performing the HASH256 (proof of work algorithm) on the exact same data, so the outcome is random. However, more powerful computers can perform more hashes per second, so they will find a solution on average, more often than a less powerful computer. These computers, by the way, are specialized hardware called miners, not general purpose computers.