Material citation and taxonomic treatment are decisive because especially the treatment includes a scientists expertise as standardized statement about these speicmen from a particular collection.
Using just publications means loosing the precision of a scientists opinion by first in many ways not providing access to the content because it is closed access, secondly because the precise links is lost because the semantics the scientists adds to an article is lost.
A scienist’s statement in an article is as follows: This specimen from collection A is part of taxon B. She makes this by using a materials citation of the specimen as part of a taxonomic treatment (which is part of a publication).