It really isn't. VirtualBox was always mediocre compared to VMWare, and ever since the Oracle acquisition, I haven't heard of any major new features or performance improvements coming out of the VirtualBox team. Though, given what Oracle has done to its other acquisitions, benign neglect is a pretty good outcome.
If you want to see the "major new features" implemented, you need look no further than the changelog. [0]
And FWIW, I've tried VMWare Workstation several times and even own a copy of it, but VirtualBox has been and continues to be the most reliable, simple method of virtualization for me (and several of my colleagues). The VMWare drivers/tools are frequently broken by kernel upgrades, the interface is clunkier and more demanding (including requiring hosts to run a couple of background daemons; VBox operates fine on hosts with just the drivers installed), and the performance, while better in some areas, is much worse in others, making it a wash for general use.