At the end of the More Meets The Eye mini series Spike said that the governments of the world agreed to give the Autobots the fuel needed to power up the Ark and send them back to Cybertron.The episode ends with Spike geting ready to leave for Cybertron but in the episodes that followed the Autobots are still stranded on Earth with no explanation about why they are still stranded and stuck on Earth.
So it's treason then!
I think they had to stay because the Decepticons resurfaced to cause more havok. After that, the Autobots couldn't just return to Cybertron yet until they made sure Megatron and the Decepticons were truly vanquished.
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Megatron not being dead at the end of the episode might have been a bit of a clue
That three parter was produced as a tie in to the comics being released if I'm not mistaken. After they were granted a whole season they had to just ignore that part at the end. After all the ark was never dug out of the Volcano.
"They have a grill, it's this grill. Now you have it...it's called America."
Trypticon did a pretty good job at trying to get it out of the volcano