N Mosfet with 8 pinout
Most likely explanation is this:

Manufacturer already owns production equipment for SO8 packages, they have performed all qualification tests, etc. Therefore, it's cheaper to use SO8. 

SO8 also has half-decent thermal characteristics if you connect all D pins to a copper plane (30°C/W between chip to leads).

Recently new SMD packages are being produced to lower inductance, offer better power dissipation, etc. But if you don't need the nifty features, SO8 is quite cheap and hard to beat.

Examples :


LFPAK is compatible with SO8 footprint (more or less)
DirectFET has great cooling and inductance but needs special PCB