Here is what I have understood from bulk plasmons.
We can show using an hydrodynamic model that they are are coupled to a longitudinal electric wave in a medium.
Thus we need to create a longitudinal electric wave in the metal to be able to have those bulk plasmons.
So, we can't excite them using light if the medium in which the metal is is the vacuum for example (bc in vacuum, the electric field is necesseraly transverse to the wavevector).
But, even if I get the global picture, it is not totally obvious for me that a transversal wave in vacuum can't convert into a longitudinal one when crossing the medium.
Indeed only the $E_{//}$ is continuous on the interface.
Why couldn't something like this happen :
We enlight the material at normal incidence.
When the light enters in it, the parallel part of the field is continuous. Thus we still have a transverse part of the electric field inside of the material. But the E_orthogonal could appear (because of surface charge appearing on the surface).
But at this point... we need the $E_{//}$ to vanish in the material to really have a longitudinal wave...
So in summary, we can't have longitudinal wave in the material using natural light because of the continuity of $E_{//}$ ?
We could imagine to have a longitudinal component inside of the material but we would need to "delete" the transverse part. And such a thing can't be done (excepted maybe in some very special material...?).
It is just to understand the link between : in vacuum with light, we have transverse E-wave then we can't have longitudinal E-wave in the bulk of a material if we enlighted it.