in tf1, megatron was found in what, 1908 or 1923 or something like that. he's found buried close with the allspark.

in tf2, the fallen finally resurfaces after 12,000 years in hiding in supposedly the moon?

in tf3, sentinel prime and his sidekicks inside the ark crashland on moon around 1960?

megatron it seems is a mastermind. he wants to use the allspark for its unlimited power and that apprently every robot creature it creates turns out to be decepticons or just plain violent. its assumed he must made a pact with sentinel prime prior to him leaving cybertron to search for allspark. since sentinel prime is optimus predecessor, therefore his ship must have been shot way far longer before megatron leaves and optimus takes up mantle for autobots in cybertron. the ship had a auto-cruise mode and was targeted to reach earth, after the ships intertial direction was set and the ships juice ran out, it just glided through space and avoided other magnetic forces from other celestial bodies but was not lucky and hit the moon instead of earth. megatron was not able to detect the ship when he left for earth because the ship was running on inertia and momentum and wasnt really on perhaps after that, which is why megatron reached earth way faster than the ark did. but megatron may have known the ark is still headed for earth prior to its losing power, but is somewhat irrelevant since the allspark is actually bigger power to harness.

so, megatron planned to get the allspark to create new decepticons and use the pillars to originally transport the new decepticons back to cybertron quickly rather than traverse the universe very long. super mega ambush blitzkrieg style.
the pillars are just a tactical war advantage rather than a source of power/energy. so it may be irrelevant if megatron still had this after getting the allspark. it just meant they didnt have to travel so far and so long back to cybertron without a 'xantium' like ship.

if im not mistaken, starscream asked megatron that suggested he didnt know sentinel prime had a deal with megatron in tf3. thus, decepticons shot the ark in cybertron but did not pursue to destroy the ship so as to obtain the precious cargo without damage. they wanted to use it to their advantage.

the allspark headed to earth by itself because it was attracted to the sun eater machine created by the fallen some 12000 years ago (self reasoning again). or possibly the amount of metal in earth which can create robots when zapped by it. whatever.

fallen went into hiding on the moon and ran out of juice there and sat in hibernation mode until they were found by decepticons when the decepticons went to the moon to steal the hundreds of pillars from the ark and also happened to find the fallen by pure chance. right. its the simplest excuse to make and is one explanation. they didnt kill and destroy sentinel prime because they know the pillars will not work without him. the decepticons have been hiding in plain sight ever since megatron froze but they knew his instructions, if the ark were ever to arrive. they should retrieve its cargo, and perhaps use the humans to their advantage (as is with human traitors in tf3) and stop anyone including humans from wrecking the ark and its contents, so they infiltrated nasa and russian space program.
the matrix of leadership while it may bring back dead robots to life, it is not limitless in power which is why fallen still wanted to use the sun machine to gather and convert sun energy into energon.
another reasoning is that fallen have been hiding all along low in power reserving his energy and therefore created his troops the decepticons to fight in his cause (their logo carries his face), and when he heard the ark had crashed on moon, he also went to moon shortly after and stayed there to oversee operations with megatron to retrieve either the allspark or find matrix of leadership to zap the sun for energon. he could not go back prior coz autobots were still abound protecting earth as was decepticons hiding trying to find clues (jetfire great great grandfather is a wheel).

so megatron really had 2 sources of power (allspark and suneater) to find to create new decepticons and transport them back fast via the pillars (avoiding headachey logistic issues of transporting millions of decepticons without enough 'xantium' sized ships).

when the 2 sources of power are no longer available to him, and with sentinel prime having his own mind. sentinel prime and perhaps megatron opted instead to bring cybertron close to earth to use humans as slaves. (cybertron is not fully controlled by decepticons yet so the wisdom of bringing cybertron to earth is confounding since autobots may have allies in humans to help fight the decepticons, whatever).