This is going to be a really tough one to answer without dipping into EU, I know, but bear with me.
According to the little bit of info we have (mainly Dark Lord, Purge, and Last of the Jedi, along with the bits and pieces we get from ROTS and ANH), Vader was obsessed for a while with capturing or killing Obi-Wan. The Emperor repeatedly downplayed Kenobi's importance, though he encouraged Vader to nurse the grudge. About twenty years later, Obi-Wan and Yoda both still seem to be really low priority for the Empire--sure, they're wanted Jedi, but nothing remarkable about the two of them other than that. Luke's never even heard of Yoda, and Tarkin blows Vader off when he mentions Obi-Wan.
Why's this a problem?
First, Palpatine knows Yoda survived. Yoda simply walked into the (presumably heavily-guarded) Senate building and held his own against Palpatine, retreating when he wasn't really defeated. Palpatine knows Yoda is superior to Mace, who gave up one hell of a fight (whether Palpy threw it for Anakin's benefit or not).
And there's Obi-Wan, who also got around Coruscant quite easily (they're smart enough to figure out he must've snuck onboard Padme's ship there). And he bested the Chosen One in combat, then snuck away right under Palpatine's nose. Definitely no slouch, either.
Throw in the fact that, sometime shortly after ROTS, the Empire would've discovered that these two reprogrammed the beacon in the Jedi Temple.
Why, then, are Yoda and Obi-Wan not tied for Public Enemy #1 on the Empire's Most Wanted list? Why doesn't everyone in the galaxy know who they are and keep an eye open for them?
Tarkin comments that Obi-Wan must be dead by the time of ANH. In 1977, we assumed he was referring to his age. But the guy would be in his late 50s at the most--still a spring chicken compared to human Jedi greats like Dooku, Mace, and Qui-Gon.