When using the Jedi Mind Trick on the Imperial Stormtroopers, there are no witnesses to see what's happening, just the Imperial Stormtroopers and our group of heroes.
In Chalmun's Spaceport Cantina, there are too many customers to mind-trick them all. And those not tricked, might notice the mind trick and not take kindly such a trick being used, turning the whole cantina against them.
On the other hand, a fight is par for the course in the Spaceport Cantina, as evidenced by the (lack off) reaction from the customers. After all, Greedo gets shot only ten minutes later and again, nobody bats an eye.
The Stormtroopers have no strong interest in the droids. They're just doing their job, doing what amounts to traffic stops. They're bored, their lack of interest means they're weak-minded in this regard, and so they let our heroes go without questioning.
Now while Ponda Baba and Doctor Cornelius Evazan seem drunk, they're not necessarily weak-minded. They're spoiling for a fight and are focused on the kid. They're angry, for whatever reason. They would need a lot more "convincing" to derail them.
Out of Universe
Of course, the out-of-universe reason is two-fold. It establishes Obi-Wan Kenobi as someone not to be trifled with. While him rescuing Luke from the Tusken Raiders could be explained as the advantage of surprise and necessary violence to save Luke, this paints him as a hardened fighter.
And of course it sets the scene for the cantina, showing what a dive bar it is by having the customers barely noticing one of hem getting dismembered.