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 (by the way, in addition to the [first 1991 publication][1] in Cahiers du séminaire d’histoire des mathématiques selected letters with updated commentaries were also published by Bru and Dugac later in an extremely interesting separate [book][2]). In letter CL (May 30, 1910) Lebesgue clearly states:

> Poincaré m'ignore; ce que j'ai fait ne s'écrit pas en formules. 


  [1]: https://mathscinet.ams.org/mathscinet-getitem?mr=1110360
  [2]: https://mathscinet.ams.org/mathscinet-getitem?mr=2406268