How did slavery become permissible in the French West Indies when it was forbidden in France?

Louis X's decree in 1315 did not abolish slavery. That's a historical myth.

First, the ordonnance did not say anything about slaves, and explicitly mentioned that it was applicable to persons "en lien de servitude", which were serfs. Second, the king did not just give freedom out of magnanimity, reminding that enfranchis*m*nt could only happen at "bonnest at convenables conditions". Finally, by explicitly saying that he hoped that other feudal lords in the Kingdom would follow his example, it was clear that the ordonnance was only meant to be applicable to those areas of France which were part of the royal domain.

_{Batselé, Filip. "The Development of a Legal Freedom Principle, Ca. 1500–1650." Liberty, Slavery and the Law in Early Modern Western Europe. Springer, Cham, 2020. 63-103.}

In the subsequent centuries France did begin to develop a national mythology of a "freedom principle", the idea that slaves are free upon entering France. Many jurists (incorrectly) cited the 1315 decree in their reasoning. We see the effect notably in Bordeaux in 1571, when the local parliament ordered a Norman slave trader's slaves be liberated, reasoning that "France, the mother of liberty, does not permit any slaves".

However, recall that France under the Ancien Régime was a fragmentary patchwork of competing jurisdiction. The freedom principle was rooted in local, civic traditions - not national law, nor necessarily supported by the royal court. In fact, in response to the events at Bordeaux, the French king in 1574 issued an order for protection of Iberian slave traders:

Citing the communities' importance to French commerce, the king offered them "special protection" from all local efforts to interfere with their families, their property, or their "servants", ordering that the "the said Spanish and Portuguese who have resided and do reside in our city of Bordeaux may freely and securely remain and continue the trade."

_{Rushforth, Brett. Bonds of Alliance: Indigenous and Atlantic Slaveries in New France. UNC Press Books, 2012.}

Thus, the emergent civic jurisprudence of a free soil principle could influence developments elsewhere, but had no direct authority beyond their jurisdiction. That included French colonies, where colonial economic interests clashed with Metropolitan humanist ideals.

In fact, slavery existed virtually from the very start of France's serious colonial efforts, around the 1620s. In New France, settlers co-opted a pre-existing system of native slavery, though this was only formally endorsed in 1709 by the Intendant Jacques Raudot in his eponymous ordinance.

As for the West Indies, slavery was integral to meeting the labour demands of the plantation economy. Initially, there were serious efforts to fill the labour needs with indentured Europeans, i.e. "temporary slaves". Importation of African slaves happened sporadically on a small scale throughout the early period, before large scale triangular trade began under the French West India Company. There does not appear to have been any true legal obstacles to either practice - the French king himself invested 3 million in the venture.

Richelieu granted a monopoly on the Senegal trade to a group from Dieppe and Rouen in 1633; there were slaves on Saint-Christophe as early as 1626; a French slave trade began to supply that island in 1633. Piecemeal authorizations for the importation of slaves from Africa were issued over the next several decades. Then, in 1664, Colbert established the Compagnie des Indes Occidentales, designed to conduct a triangular trade that included the transportation of enslaved Africans to the Antilles. This was the beginning of the real French slave trade.

_{Miller, Christopher L. The French Atlantic Triangle: literature and culture of the slave trade. Duke University Press, 2008.}