Because your wiring is a 'spider web'.
The 3 pairs of wires go to three different places.
Two of them go to other jacks, or where other jacks 'used to be'.
The other pair goes "towards" where the phone line comes into your house. Maybe 'directly", or may to another jack that is 'closer' to the entry-point.
You need another pair of wires from the telco entry-point (or wherever the 2nd line originates) to that jack. connected to the black/yellow terminals.
Of course, this implies that you _have_ a "second line", from the telephone company, or 'somebody else' (e.g. a VoIP provider).
Then, obviously, you have to have a "two line capable" telephone instrument, or some sort of a 'switch' (as in a simple mechanical "DPDT" device) between the phone and the wall jack.