Ok, we know why--more people, more advertising, etc.
But JupiterResearch's David Schatsky points out a little tidbit from from his company's recent research. Namely, that "the share of consumer online time spent on Google properties is disproportionately small compared to Google's share of online ad revenue. Google Finance aims to change the equation."
That explains even more Google's reason for becoming a portal. (The consequences of which Jeff Jarvis warns us about today).