Because this is about as petty and unsportsmanlike as it gets, without being particularly dangerous.

There is nothing about red cards about "sportsmanlike conduct". And either way it is NOT unsportsmanlike. This is a defender trying to stop the best fucking player in the world. Cynical fouls are literally written into the rules and are considered a good defensive play if there are no other better methods to defend it.

But it comes at the cost of playing on a yellow card. This means a player could get a red, getting risked of substituted out early, risk missing future games, etc.

It's an accepted part of the game, always has been. The only things that affect red cards are "violent conduct" (elbows, punches) and "excessive force", which in the rules is constantly referring to studs up challenges to the knee area.

Pulling someone to the ground with your hands is not quite excessive, it's necessary to stop the play. If he picked Messi up and supplexed him, then you've got an argument.

If you want to take about "sportsmanship", let's talk about the topic of diving. But not this.