The amount of space available is limited. An early decision was made to increase the number of channels in a zone, so this resulted in a reduction in the number of zones.
The commonly used policy of using one zone per repeater is not applicable to OpenGD77. Only one channel per repeater is required. To make best use of the firmwares features it is recommended to re-arrange existing codeplugs. This will normally result in needing fewer zones. In fact you don’t need to use zones at all if you use the ‘All Channels’ feature.