The prohibition of consuming of grapes was used as a hedge to prevent even the desire to drink wine
The prohibition of fermented drink and wine could be because this was a highly intoxicating wine which could somehow have lead to defilement on the part of one who had just taken a vow.
Leviticus 10 NKJV
9 “Do not drink wine or intoxicating drink, you, nor your sons with you, when you go into the tabernacle of meeting, lest you die. It shall be a statute forever throughout your generations, 10 that you may distinguish between holy and unholy, and between unclean and clean, 11 and that you may teach the children of Israel all the statutes which the LORD has spoken to them by the hand of Moses.”
John Gill commentary
And the Lord spake unto Moses,. At the same time, or immediately after the law concerning the woman suspected of adultery was given; with which the following law concerning Nazarites may be thought to have a close connection, as some Jewish writers observe, women being concerned in it as well as men; and as wine leads to adultery, as Jarchi observes, abstinence from it, which the Nazarite's vow obliged to, and forbearance of trimming and dressing the hair, and a being more strictly and closely devoted to the service of God, were very likely means of preserving from unchastity, and any suspicion of it
Seeing that the nazarite presented a state of innocence so he was prohibited from consuming anything that could have lead to defilement