For aggregation to occur, you need an option that stands significantly out from the crowd for an extended period of time.

It needs an "edge" that draws incremental users into it instead of other options.

By being prominent it becomes a schelling point.

If there is a network effect to the option, then its edge can increase as more people join in, becoming a runaway gravity well effect.

In a truly open system, it's harder for aggregators to emerge.

There's no obvious "edge" to get the compounding loop going from.