It's by no means the least popular first move for White. In fact, it's often listed as the third or fourth most popular move, since such stalwarts as 1.h4 and 1.a4 are beneath almost all consideration. Ranking above it are 1.e4 (the King's Pawn), 1.d4 (the Queen's Pawn), and 1.Nf3, which I used to call the Reti since my tutor told me to, though I have no accurate name for it now. On some lists it will exchange places with 1.Nf3, though I always thought of it as the 4th most popular.
As for reasons? The first is the type of game. The English Opening, being different from the King's Pawn, will naturally lead to different kinds of positions. Some people might be able to grit their teeth and sit through the strategic manuevering of the Ultra-Symmetrical Variation of the English (1.c4 c5 2.g3 g6 3.Bg2 Bg7). Some might strive to unbalance the situation right away in the King's Pawn with the King's Gambit (1.e4 e5 2.f4).
That isn't to say that the English Opening is a game where manuevering is king and there is little else; even with the same -style-, one might not know or like the positions that the English gives them, but enjoy the peaceful development or useful transposition that the Four Knights Game gives them rather than go straight for the jugular with a gambit or razor-sharp Traxler.
Since I've spat out paragraphs and paragraphs (and had still more written, but deleted them since I misread the initial post), I'll just end it with this: don't believe all of the statistics you see.
For some reason, chess programs like chessmaster like to use the english opening. I looked at many databases and found that although it is the least popular first move for white, it has the most win percentage. So why use king's pawn?