how come they couldnt re-develop the first way
Only part of hyperspace travel technology was rediscovered. There are still many missing pieces. What has been compiled led to the first discovery of phase space and travel using it.
and why does earth have to dissapear?
Earth never disappeared, it was just forgotten as the developed world it once was. The planet surface has been ravaged by nucleostatic warfare, and while the geography remains largely status quo, most of the ecosphere and the edifices of civilisation were razed. In the modern Sins universe, Evertha is just some petty terran planet with no real significance. Alux is presently considered to be the homeworld of the human race while Helms Kol is the de facto capital of the Trade Order.
why does it always have to dissapear?
It's status as the social, economic, cultural and political center of the galaxy was simply too great for warring factions to ever ignore it.
The Undying. 
