Obi-wan is a fairly distinctive name, and Vader knew he had survived Order 66.
He could hardly call himself Obi-wan Kenobi any longer. That said, he needed a link to his previous name, in the event that he was needed (as he was, by Bail Organa, years later).
Last names, in the SW galaxy as well as ours, tend to be firmer - a person's last name rarely changes outside of big events (such as marriage). First names are much more malleable.
Common names are also more frequently used alone. He wasn't even usually called 'Ben Kenobi' - he was called 'Old Ben' most commonly.
'Ben' is a close relative, phonetically, of his given first name, but common enough to not raise eyebrows. If his name DID filter into official communications somehow (such as in an official incident report) he would likely have been referred to by the citizenry as "Old Ben" or 'the hermit'.
His seclusion and the relative unimportance of Tatooine helped him hide behind a relatively flimsy false name, while still remaining locatable to his allies.