Claude Piron, a very famous Esperantist, wrote an essay about this very topic. He provides the following “underlying anxieties” under some people’s uneducated attacks against Esperanto:
- Avoiding change in the status quo
- Language as a cared value and sign of identity
- Fear of risk
- Fear of direct contacts (inability to hide behind a language barrier)
- Fear of infantile regression (“It is too simple to express complex ideas”)
- Fear of inferiority in connection with facility (they think coming up with a more complicated solution would prove themselves to be intelligent, etc.)
- Fear of heterogeneity/fragmentation anxiety
- Fear of lowering standards and destruction
To Most if not all of the criticisms that arise from these are not based on actual facts about the language. I highly recommend reading the essay.