There is some evidence to suggest that the Earth was in fact destroyed by the Vogons under the guise of 'Building a hyperspace by-pass' five minutes before the crucial readout of the Ultimate Question of 'Life, the Universe and everything', to which the answer is famously documented as 42 (as calculated by Deep Thought, the designer of the Earth). It is entirely possible, in this historians point of view, that the order for the destruction of Earth was signed by non other than Zaphod Beeblebrox, the President of the Galaxy; although it is entirely possible that he did not realise that was what he was signing, instead believing he was signing pictures of himself "To myself, with frank admiration".

Sources: Some stolen Magrathian Construction Plans The Scientific Papers of a couple of mice I bribed with cheese and a slot on the 'Tonight show' And some guy I met at a party that served exceedingly good Pan Galactic Gargle Blasters

It was to make way for a hyperspace bypass. This was particularly controversial, since hyperspace travel was more or less obsolete by the time Earth was demolished.

However, due to the plural nature of the sector containing Earth, it was not destroyed in all realities and thus I'm able to write this from a home in Wimbledon.