Setting a specific Block Number is predictable because each block takes ~13sec (approximately) to mine, so why was Terminal Total Difficulty (TTD) chosen to trigger the Merge?
ETH Merge – Why Does ETH Merge Use TTD Instead of Block Number?
the-merge
Related Solutions
Each block has a difficulty.
totalDifficulty is the sum of all the difficulty of the block and all previous blocks. Another way to describe totalDifficulty is "cumulative difficulty", but cumulative is a longer and not necessarily simpler word.
Even if hashrate decreases, difficulty is still larger than 0. So totalDifficulty will keep increasing and eventually exceed TTD.
The Beacon Chain is the Proof of Stake consensus layer. Conceptually, the execution chain gets merged with the Beacon Chain:
Source: https://github.com/ethereum/annotated-spec/blob/master/merge/beacon-chain.md#introduction
Best Answer
Because in the PoW network, the canonical chain represents the chain with the highest amount of work, not highest block number. If we use the chain with the highest block number, then a minority chain with a lower hash power can reach a certain block number faster and hijack the transition to the merge upgrade. That is why it's important for the PoS chain to pick the PoW chain with the highest amount of work, and that is determined by the total terminal difficulty, or TTD.