Why is that? A penalty is given for any sort of foul, even if it's not a goalscoring opportunity.
Here, I think it was a foul but not a clear-cut goalscoring opportunity, hence a penalty yes, but not a red card.
Ah right I see. Well in that case we disagree, I think he denied him a goalscoring opportunity.
Red Card was for:
Denying an obvious goalscoring opportunity to an opponent moving towards the player’s goal
by an offence punishable by a free kick or a penalty kick (known as a professional foul)
Therefore, I conclude , that as he was the last man, and behind the striker, it was an obvious goalscoring opportunity, therefore punishable by a red card.