Why do the Republicans and Democrats hold, generally, the same primary schedule?

[No War, but the War on Christmas]

I'm just wondering if anyone can answer this, cause I know it hasn't always been the case. At which point did the primary schedules for both parties converge and was there some formal agreement or did it just generally become that way out of convenience or something?

[SteveRogers]

Since under the modern system the state governments actually have to put on the primary elections, it’s much more cost-effective to hold both primary elections at the same time. This, if a state legislature decides to move the primary date, it will effect both parties’ schedules.

[Famous Mortimer]

Because primaries are organized by state governments, not by the national party.

[Nyvin]

Because it's a lot cheaper.