Why don't we see business jets with wingtip fuel tanks anymore?
One of the main reasons wing tip tanks were used was to counteract some of the forces of wing bending by adding mass to the end of the wings. As time has progressed and our understanding of materials has grown as well as our ability to design structures better the need for tricks like this has gone away.  

As our general understanding of aerodynamics has gotten better so has the understanding of the impact of wing tip shape on reducing drag. As such wing tip tanks have been traded out for wing tip devices.

In some cases the tip tanks are an option available as a later add-on or at purchase time. Generally this increases the range of the aircraft at the cost of useful load. As such, it may just be that most people who order planes these days prefer the higher useful load and the reduced range, rather than the extended range flying with a lighter load.

You can find some more discussion on it here.