The Latin Iesus is an irregular form of the 4th declension. (The Latin declensions are like verb conjugations in Spanish, but applied to nouns).
Iesus is in the singular nominative case: the "name" of the word (as seek it in the dictionary) and the form it takes when is grammatical nucleus of subject. Iesum is in singular acusative (like direct complement). And Iesu is the form that takes all another cases (dative, genitive and ablative singular and plural).
Since the formation of the Spanish language until the mid-20th century all Spanish speakers say the Latin name of Jesus Christ weekly in a ceremony: Mass. Consider the following passages of Mass in Latin:
Grátia Dómini nostri Iesu Christi, et cáritas Dei, et communicátio Sancti Spíritus sit cum ómnibus vobis.
...exspectántes beátam spem et advéntum Salvatóris nostri Iesu Christi.
Dómine Iesu Christe, qui dixísti Apóstolis tuis...
In the simplified version of the Roman Missal of St. Pius V in 1570 I count 3 mentions of Iesu and none of Iesus or Iesum. In a full version of modern Latin Mass I count 10 mentions of Iesu, 5 Iesum and none of Iesus. (It's an approach, the text changes daily).
Consequently, we can say that there is not really a loss of -s sound in the Spanish word "Jesús". What we have is a Latin cultism that led to the synthesis of two words in one. Iesu Christi became Jesucristo.
Until the eighteenth century remains the original orthographic form with two words separated by a space, by example this Spanish book titles:
But in the late eighteenth century there is evidence that the words are coming together in one compound word. A hyphen shows the first step of this join:
In 1790, an edition of a book prefers to use the hyphen, and 27 years later, the new edition of 1817 already uses the modern form (and has even lost the H):
Then we could say that the current Spanish form Jesucristo is used since circa 1800. A good evidence of this transition is found in this book
in which among the hundreds of references to Jesuchristo, appear hidden 3 times Jesu-Christo with an hyphen.