Why does only the earliest transaction matter for double spending?
"Doublespend" refers to an attempt of spending the same funds twice. In Bitcoin specifically this occurs when a user publishes two transactions that are in conflict due to attempting to use the same unspent transaction output as input. Obviously, only one of the two transactions can be valid. 

Before Bitcoin was published, the solution to the doublespending problem was to designate a central actor that decided which of the two transactions would take precedence. The central actor would do this by announcing which of the two it had seen first.

The "first-seen" behavior was at first explicitly encoded in node behavior: nodes would not accept a conflicting transaction into their mempool (the temporary storage of unconfirmed transactions). However, this lead to the incorrect assumption that zero confirmation transactions could be reliable whereas actually the doublespending problem is solved by miners collecting transactions into blocks for confirmation. The blocks may only contain one of the two conflicting transactions, as otherwise the block is invalid. Thus, the blockchain specifies the precedence of transactions by converging on a common journal of the transactions without a central actor.

In the past months, we've seen the "first-seen" paradigm get softened significantly as doublespends (of unconfirmed transactions) have become more or less trivial due to the high demand for blockspace. As it is becoming common for users to update transactions with a higher fee (i.e. a doublespend), the "first seen" paradigm is falling out of use and being replaced by the defacto mechanism of "first-included". Assuming rational miner behavior selecting the most profitable transactions first, we will converge on a full replace-by-fee system eventually.

In other words, when the whitepaper describes the requirement for the Bitcoin system to create a convergent history with "first-seen", it describes a simplified situation that didn't account for the network dynamics of competition for blockspace. From today's perspective this should be understood as "first-confirmed".