I haven't used Fabric before, but from what I've heard:
Forge uses an event driven system. Fabric (IIRC) treats all mods as coremods (mixins).
This makes Fabric significantly more lightweight than Forge, and therefore quicker to develop and update. I wouldn't be surprised if a Forge update's workload is more than 10 times the workload of a Fabric update.
However, since Fabric mods use mixins, it is very easy to cause incompatibilities between mods (think of Forge coremods), while Forge's event system can theoretically supports infinite amount of mods.
Apart from that, Forge has a much bigger user base than Fabric, therefore the Forge team has to respond to more PRs and issues. I imagine this to be very time consuming.