The 2015 Mid Year Report on US Charitable Giving shows that one major factor is the rise of new, focused, niche nonprofits.
"2015 first half U.S. charitable giving has been exceptionally (and unexpectedly) strong – up a total of 7.6% to $238.88 Billion from $222.03 Billion for the same period in 2014. Most of this strong giving performance is attributed to the continually good news of improving employment rates. Individual donors, who account for 74% of all U.S. giving, are finding jobs, less fearful of losing their jobs, and enjoying some splendid economic conditions like low interest rates, low gas prices, and little inflation. This leaves them with more disposable income that can be applied to charitable giving." --the Atlas of Giving