Why A series circuit provides only one path for charge to flow, so every electron that leaves one component must pass through the next. That means the same current flows through every component in the chain, while the source voltage divides among them in proportion to their resistances.
Watch out Parallel is the opposite case: the voltage is identical across each branch while the current splits among the branches according to each branch's resistance.
Series = Same current, one path. Parallel = same voltage, current splits.