Because Pochettino explained why - which matches with what Sissoko says - that it was due to his training not being at a level he expected. GK has said the same - Poch expects his team to train to a certain level in order to play. Sissoko wasn't and he explained as much at the time.
I trust the combined quotes of the team, not a translation of French that is used in a click bait article.
We've seen what Poch did to Townsend (and others) - he'd hardly drop Sissoko for just one game if there was an issue.
So the quote is an invention? Also L'Équipe report that Sissoko and Poch are barely on speaking terms and the player can't wait to get away.
Whoever you believe, it seems pretty clear that he was a long way from being the 'model professional' you claimed. If he wasn't training at the level required how could he possibly be?