It's less verbose than Symfony2 and, being pretty opinionated, requires much less configuration for new applications.
Facades, macros and route closures supports rapid prototyping and are pretty easy to replace as needed. Included libraries like Flysystem, Cashier, Elixir and the new cron wrapper also eases rapid development.
There are also a few tools around it, like Homestead and Forge, that makes it easy to set something up quickly.
It's not like you can't do rapid prototyping with other frameworks, but Laravel, to me, is the first PHP framework that actively supports it while providing a flexible enough structure to grow in.