Well the answer is simple because nabid النَّبِيذَ or an-nabidah is a drink or a wine made with fermented dates or grapes put into or mixed with water (or some water mixed with dates/grapes in different sedateness states, which were left for a while ... even until they get fermented)!
النَّبِيذُ : شُرابٌ مُسكر يُتَّخَذُ من عصير العنب أَو التمر أَو غيرهما ، ويُترَكُ حتى يختمر والجمع : أنْبذَةٌ
Therefore our Messenger (peace be upon him) told the sahaba (May Allah pleased with them) to use instead of fermented dates: grapes or dates or unripe dates without mixing as when they are mixed one couldn't distinguish by taste whether it is fermented yet or not! So one would drink alcohol without being conscious of doing this! For this reason they have been asked to make a drink from non mixed fruits!
I could only find an Arabic Fatwa and explanation supporting my Answer so far!
And Allah knows best!