And what exactly were Bail Organa and Obi-Wan doing in all that time, hmm?
It makes sense that they kicked off the Rebellion immediately, but didn't make any overt moves at first, since those moves would have been expected. They would have spent the first few years regrouping but within, say, five years, they should have regrouped well enough that their Rebellion would be worthy of the name at least. Powerful people who are accustomed to being influential and getting their way, who see their cherished way of life and ideals (not to mention their power base) overthrown, don't just sit on their asses.
The twins would be part of a long-term plan. With Leia handed off to a politically powerful family and groomed for leadership, she seems like Plan A. When the Force kicks in for her, she'll already be a respected leader of the Rebellion.
Luke wasn't trained or groomed for anything, and Obi-Wan didn't even pay attention to him till Luke came to
him because of a totally random event, and it's extremely odd that he was "hidden" with Anakin's own family. Bail and Obi-Wan might have figured Vader would think Padme had only one child - and Vader might have suspected all along who Leia really was - and therefore he wouldn't be looking for a second child anywhere, much less on Tatooine of all places. Luke was Plan B in case his much more high-profile sister, who was in far more danger, was killed.
But if Lucas tried to write the roots of the Rebellion story, I'm sure he'd frak up the political verissimilitude, because he didn't manage to create anything approaching a politically believable landscape in the Prequels. There's a difference between declaring that this or that happens, and actually selling it to the audience.