Because he
spent the day in Quincy, Illinois, where the Mississippi River is about to flood its banks, helping to pack sandbags and organize volunteers. Because he's both turned his campaign page into a donation site for victims of the Midwestern floods and tornadoes and, perhaps even more importantly, used his vast organization of volunteers to send out local emails and give precise, specific directions and maps about where to go to help in Iowa and Illinois and Indiana and what supplies to bring. (Bottled water is especially useful because most of the reservoirs and water supplies for these towns have become contaminated and are undrinkable now.)
The National Guard isn't able to help at its normal levels in Iowa, where two of the largest cities, Des Moines and Cedar Rapids, have been largely evacuated and are facing tens of millions of dollars in property damage (though thankfully little loss of life so far). Why isn't the National Guard helping? Well, because 50% of those troops are currently serving bravely in Iraq and Afghanistan. Floods are precisely the sort of situation where having a large body of quickly mobilizable fit men and women and top-notch equipment make a huge difference.
The White House is not mounting an organized response or sending in federal assistance at this point.
So I'm glad that the person with probably the best organization of young able-bodied volunteers in the states of Iowa and Indiana is using that resource to help folks in those states, because, well, they need it.
Pray or hope for an end to rain and tornadoes here in the Midwest, and let that rain come to stop the fires in Northern California and the heat wave on the East Coast, which is ironically killing far more people than the floods here.