It has nothing to do with the conflict with Borel which developed later, and one can find a pretty explicit answer in the aforementioned letters of Lebesgue to Borel.
(These letters were first published in 1991 in Cahiers du séminaire d’histoire des mathématiques; selected letters with updated commentaries were also published later by Bru and Dugac in an extremely interesting separate book.)
In letter CL (May 30, 1910) Lebesgue clearly states:
Poincaré m'ignore; ce que j'ai fait ne s'écrit pas en formules.
(“Poincaré ignores me, because what I have have done is not written in formulas.”)
