Because Kabuto tampered with the Hōzuki DNA, creating his own execution of the clan's Hiden by fusing it with another technique. If Kabuto would've simply injected himself, and not altered the properties he would've very well been immune to Genjutsu in the same manner the Hōzuki are.
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Kabuto very well could be immune to Genjutsu, but the the execution for his Hōzuki-altered technique isn't as useful as the standard Hōzuki technique in regards to actually combat. Kabuto's altered version is more so for tactical retreat, whilst the Hōzuki Clan's technique can be used for both: Assault, and retreat.