Elegance. It's Occam's razor. If we can do with only one field, it's probably it.
It's inductive and abductive reasoning. The one field, and it has lot of mathematical characteristics which makes it unique on its own, and also it is the only one that has a chance to fit, is the e8 field popularized by Garrett Lisi.
If a universe were to be designed based using the e8 Lie algebra as an elemental field, it would look a lot like our universe.
Currently the standard model is a patchwork of field added as experiments for observing particles were possible to realize. The big picture's view is a unified theory which fits perfectly all existing data.
Occam's razor has nothing to do with this, it only applies once you have multiple competing theories - you can't use Occam's razor to decide that a theory "should" exist.
Currently, we don't have any theory that works that's any simpler than the SM. So that's the theory that Occam's razor currently tells us must be true, as it's the simplest alternative that actually works.