Question

Why is blue so hard for speech recognition to recognize?

Answer 1

different people (women, men, children, elders etc.) have different voices

sometimes the same person sounds different for example when the person has a cold

different background noises

everyday speech sometimes contains words from other languages (like you have the german word Kindergarden in the US/English)

some persons not from the country itself learned the language (they usually sound different)

some persons speak faster, others speak slower

quality of the microphone
etc.

The problem is not simply one of signal analysis, but there is a large natural language component as well. Most of us understand spoken language not by analyzing every single thing that we hear, as that would never work because each person speaks differently, phonemes are suppressed, pronunciations are different, etc. We just interpret a portion of what we hear and the rest is 'interpolated' by our brain once the context of what is being said is established. When you have no context, it is difficult to understand spoken language.
hope this help to you