One thing that is at work here, is the will of a higher power. Gandalf says of Bilbo finding the ring,
"Behind that there was something else at work, beyond any design of the Ring-maker. I can put it no plainer than by saying that Bilbo was meant to find the Ring, and not by its maker."
Later, when Frodo flees Boromir on Amon Hen and puts on the ring, he is in danger of being seen and destroyed by Sauron,
"Then as a flash from some other point of power there came to his mind another thought: Take it off! Take it off! Fool, take it off! Take off the ring!
The two powers strove in him. For a moment, perfectly balanced between their piercing points, he writhed, tormented. Suddenly he was aware of himself again. Frodo, neither the Voice nor the Eye: free to choose..."
I interpret this passage this way: Sauron senses Frodo wearing the Ring close to Mordor and starts looking for him. Gandalf, who just perished in Moria, is with Iluvatar about to be sent back into Ea. The Voice is Gandalf, probably moments before being sent back, urging Frodo to take off the ring and striving against Sauron. Note that they are evenly matched. But then, suddenly, an omnipotent power frees Frodo from this conflict and restores to him his free will, and I believe this is the work of Eru. Likewise, I think it was Eru who guided Bilbo's blind hand to the Ring beneath the Misty Mountains.
Gandalf says about Bilbo and the ring:
"Among the wise I am the only one that goes in for Hobbit-lore: an obscure branch of knowledge, but full of surprises. Soft as butter they can be, and yet as tough as old tree-roots. I think it likely that some would resist the Ring far longer than most of the Wise would believe."
There is also the profound difference in their personalities. Bilbo is gregarious, compassionate, generous and selfless. In his adventure at the Lonely Mountain, he was willing to give up all of his treasure to make peace between his friends and the elves and the people of Laketown. Smeagul on the other hand, right away takes to murder and thieving.