why is Lobo the last of his race.?

being the individualist that he is, he designed a killer virus to wipe out his whole planet leaving him free to take up bounty hunting for fun and profit

Both goodlordhoratio and wrathpuppet are correct - however, Lobo did indeed meet up with one other of his race who survived his holocaust - his fifth grade schoolteacher! I DO have that miniseries that wrath is talking about, and I remember that he was commissioned to bring in the schoolmarm ... without harm! This lent itself to great hilarity as Lobo never breaks a contract (or his word) although he does like to live up to the LETTER of said contract, instead of SPIRIT. The schoolmarm sure made it difficult on poor Lobo - until she was delivered safe and sound to the head of L.E.G.I.O.N. whereupon the safe delivery and thus, the contract fulfilled, he promptly snapped her neck. And indeed, Lobo is the last of his race with that heinous fraggin' action!

goodlordhoratio is correct. However, there was a mini-series in which it was revealed that he was not the last of his kind. One of his schoolteachers survived the massacre.

Unfortunately, I don't remember what happened to her at the end, but I would assume that he killed her, solidifying his standing as the last.

coz Lobo had eaten them all