There are a lot of threads on this... But for a synopsis.
MBP uses LPDDR RAM, I don't believe this is currently supported in the DDR4 specification? So they use fast low power memory, in order to maximise battery life of the machine. Generally laptops with 32GB of RAM use DDR4 and have about a 2 hour battery life. Apple felt this was unacceptable, and given the niche requirement of 32GB of RAM decided to omit it. I believe Coffeelake chipsets will be able to support 32GB however, so we may see something next year or after. Otherwise nothing soon.
Of course there's the other 'opinionment' about Tim Cook, Apple going down, Sir Saint Jobs etc.
But that's generally speaking the reason. Loads of threads if you use the search function, you can peruse at your leisure.