Because well-written content and thorough investigation has to be paid for somehow. If it's worth reading, it's worth paying for IMHO. (I assume you do not work for free, either.)
It's newspapers and the decline in print.
Newspaper death throes.
Because of adblock
Because of malware delivered through unblocked ad networks.
(FTFY)
Regardless of your reasons, the end result will be more paywalls or entire articles that are actually ads (this is what yahoo does now).