[ talking of BeCu: ]
BeCu has the combination of very low resistivity and good spring properties, and is easy to machine/manufacture as it can be stamped and pressed in annealed state and then heat-treated to develop full strength.
An approach in large switchgear is to use steel springs with flexible copper braid to carry the current to the contact - use each metal to its best advantage, and avoid expensive alloys - but that's no so easy to do in small fittings and connectors.
But not low resistivity, and many spring metals are not corrosion resistant either.
BeCu has the combination of very low resistivity and good spring properties, and is easy to machine/manufacture as it can be stamped and pressed in annealed state and then heat-treated to develop full strength.
An approach in large switchgear is to use steel springs with flexible copper braid to carry the current to the contact - use each metal to its best advantage, and avoid expensive alloys - but that's no so easy to do in small fittings and connectors.