Franky is the main suspect because he has some extraordinary skills and even knows about the ancient weapon blueprint. Still, the world government doesn't consider him a threat, and in addition wanted poster containing Sunny ship, not Franky, so there is no life threat, and he has been observing since Gold Roger's time, so at last, maybe he will show his identity as a Cipher Pol agent, as we saw the same thing when the mayor got shocked when he found his own workers were government dogs.
