because common names are only common to certain areas.
for example the black skirt tetra is a US common name for Gymnocorymbus ternetzi
but here in the UK it is known as black widow tetra.
using scientific names and or L, LDA (plecos), C (corys) and most recently TW numbers
(new undescribed danionins) for fish means that everyone
worldwide knows exactly what fish is being described.
The other huge problem with "common names" is that far too often they are used multiple times...for example Rasbora borapetensis has at least 3 common names at least two of which that are shared with several other Rasbora species.