脖 is rarely used alone in modern speech; the full word is 脖子 (bózi). 脖 functions as a bound morpheme, appearing in compound words like 脖颈 (nape of the neck).
⺼ is the flesh-meat radical (left-side form of 肉). The neck is one of the body's most exposed muscle-and-skin junctions, so it naturally takes the body-part radical. Compare 脸 (face), 腰 (waist), 腿 (leg) — all anatomy under ⺼.
孛 (bó) supplies the sound, unchanged. 孛 is rare standalone today (it once named a kind of brightly flaring comet). Its main job is to drive a phonetic series: 脖 (neck), 勃 (vigorous), 渤 (Bohai Sea), 悖 (contradict).
