Three runways (or pairs) 60 degrees apart was very common. Where land was limited, two runways (or two pairs) at 90 degrees was almost as good. These generic designs could be built very quickly almost anywhere and work well regardless of the prevailing winds, which was particularly important when building hundreds of them in a hurry during WWII.
However, the needs are different. Modern jets need longer runways than WWII planes but aren't as sensitive to crosswinds. And there is now decades of data on which runways were used the most at each airport, so only those were extended.
At the same time, traffic has grown, and building new runways parallel to the main one(s) adds capacity faster than it adds costs, so it makes sense.
Some airports kept the intersecting runway(s), but it(they) don't provide anywhere near the capacity now, still cost a lot to maintain and take up valuable land that could be reused for fancy new terminals or hangars, so many airports have ripped some or all of them out.