Why didn't Obi-Wan use the Force in the Cantina?

Question

In the Mos Eisley cantina in Episode IV, Luke is picked on by a couple of random thugs (at 1:37 in this clip). Obi-Wan tries to defuse the situation by offering to buy them drinks, but they turn violent, and Obi-Wan has to use his lightsaber, thus attracting Imperial attention that he was keen to avoid.

Why didn't Obi-Wan use the Force on these thugs to make them change their minds? Or if he did, why didn't it work? They look like exactly the kind of "weak-minded" person who would be vulnerable to it, especially as the scarred human seems to be drunk.

Answer 1

Witnesses

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.

Weak-mindedness

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.

Answer 2

Obi-Wan does appear to attempt to use the Force to defuse the situation. However, the situation calls for a bit more subtlety than his previous interactions with the stormtroopers, and his efforts are (obviously) ultimately unsuccessful.

The first thing Kenobi says to Dr. Evazan and Ponda Baba is: "This little one's not worth the effort." This mirrors the way he talks to the Imperial stormtroopers outside—largely making statements to them that they accept as the truth. ("You don't need to see his identification." "These aren't the droids you're looking for.") With so many people standing around the cantina to witness the events, Obi-Wan may feel that directly telling the two aliens who are hassling Luke, "You aren't going to bother him any more," might seem peculiar—and he is, after all, trying to avoid attracting unnecessary attention. So he tries to use his Force ability to convince Evazan and Baba that Luke is indeed, "not worth the effort." He then follows that up with an offer to buy them a drink, which both makes the interaction seem more natural and proffers another way to get out of the altercation.

However, neither Obi-Wan's Force power, nor his offer of to buy Evazan and Baba drink work out. Whether Evazan is being truthful when he says, "We're wanted men. I have the death sentence on twelve systems," is not entirely clear. However, he and his partner are clearly resourceful survivors—and thus not the kind of individuals that are easily susceptible to Jedi manipulation. Just like Jabba the Hutt in Return of the Jedi, they are unaffected by the Force compulsion.