Maybe they figure it'd take too long to exhaust all the letters (assuming both teams are equally adept). You'd have each team sitting at 7 and trying to pull hard letters back and forth all night long.
Once you've played a few times I think you'll see why it's 8 instead of 9. In many instances, both teams have 6 or 7 and there can be a lot of back and forth in the end game. With 9 the end-game can be end-less!
Having left the rules behind and playing with family, I taught the game that it was first to capture nine or more. We got a close game that was 8-7, but neither team could break through because each play would pull the letters back to their side.
It got dull and boring in a hurry. Goal of eight means you can't just play a prevent defense in the end game, you actually have to go for the win.