When a wheel is built, spokes need to be laced right, then tensioned, wheel trued, then spokes need to be stress relieved, then, after a few rides, check for any loosening, or wheel coming out of true. When that is all done well, all the spokes take wheel loads equally and it's all fine.

From time to time, check for loose spokes. If some come loose, tighten them again, true the wheel and ride on.


However, here's what I believe was the scenario with your wheels:

If loose spokes are not checked, the adjacent (or opposite side, whichever) spokes will take more load, getting fatigued. Once a most fatigued spoke brakes, it can be changed, but it wasn't the only spoke that took extra tension, some other spokes are near their breaking point as well. If more than one spoke were loose, not tight enough, than probably most spokes on the wheel have come near their breaking point.

So when you change a broken spoke on wheel that went through that scenario, it is only a matter of time before another spoke breaks. It will go on and on until you replace most of the spokes - one by one. In addition to that, the new spokes, once another spoke breaks, also take more load and it is arguable how well the load distribution can be done with the combination of new and worn spokes.

Long story short: the best option (IMO and experience) is to re-lace the whole wheel with new spokes. If rim and/or hub are not in the best condition, change them as well. Wheel well built with decent quality rim, hub and spokes can last for decades - just check the spoke tension from time to time.