Hamas has some authority to form a government. Abbas has none.
It's not any of the obvious stuff involving the fact that the Palestinian Authority is a terrorist kleptocracy whose very existence is tied to making constantly escalating demands in order to sabotage peace negotiations that if successful would eliminate its reasons for existing...
...this is a more direct legal approach dealing with Abbas' authority to negotiate on behalf of the so-called Palestinian people considering that he hasn't stood for elections and the legislative authority is back in the hands of the PLO Council.
Legally Hamas has some authority to form a government. Abbas has none. The pretense that Abbas is the legal leader of anything is being maintained by Western diplomats who don't want to accept the end of the peace illusion and are continuing to illegally deal with Abbas.