It was the limit of manufacturing capability at the times of release.
The first Pi was released April 2012, the B+ was released July 2014, and the Zero was released November 2015.
In 2012, Broadcom could only make the SoC well enough that the ARM1176JZF-S could reliably reach 700MHz. Some units could be overclocked with good results but many could not.
By the end of 2015 - almost four years later - they had improved the precision of the manufacturing process so that 1000MHz was possible and reliable on all chips.