In Old Spanish the form was Ihesu Xpisto (pronounced iesu cristo1,
[je.zuˈkɾis.to]), transcribed from the letterforms of the Ancient Greek Ἰησοῦς Χριστός (Iēsoûs Khristós).
So in the transliteration from the Greek, there is already apocope of (both words') final consonants.
The Latin form itself, Iēsus is declined in the genitive as Iēsū.
So there present a number of reasons this may have happened:
- Normalisation of the Old Spanish phrase Ihesu Xpisto to Spanish orthography.
- Use of the genitive form of the Latin.
- Syncope/apocope that occasionally happens with the first word in Spanish compound words, e.g.
o y bajo > altibajo
e y uno > veintiuno
o novio > amigovio
a abajo > bocabajo
1. yesu / iesu / hiesu - y has ambiguous value as a consonant, depending on Spanish dialect (sheísmo, zheísmo etc), though it has precedent as being used to transliterate semi-vowel/consonantal initial i before e:
Yécora (en euskera Ekora o Iekora y oficialmente Yécora/Iekora)