Because it is little more than a tax-exempt travel agency and a conference organizing business. According to their own IRS 990 tax filings, in 2012 and 2013, which are the last two years filed, the organization spent exactly $0 on the "better health care to the impoverished" that you speak of. While at the same time $4.8 million was spent on office supplies and $9 million on personal travel for the Clinton family and foundation staff. According to their 2013 tax filings about 10% of the Foundation's donations received actually went to IRS qualified charitable causes. 33% went to salaries, including salaries to both Bill and Chelsea Clinton, and 34% went to miscellaneous expenses such as publicity and "fringe benefits" to staff. So when the Clintons say that 88% of the funds "go directly to our life-changing work", they are lying, or use a broad interpretation of what qualifies as "life-changing". Either that or they are lying on their Tax returns.
So anyone with the common sense of a 10-year old can see that their foundation is a complete fraud and has been for a number of years. It's been little more than a personal travel and lifestyle fund for their family, close associates, and political allies. And Oh by the way, they've done so with out paying any taxs. The IRS is responsible for auditing the 501(c)3 status of a charity. If a conservative 501(c)3 organization was reporting on their Form 990 that they were only giving 10 cents on every donation dollar received to an actual charitable cause, they would get their 501(c)3 status yanked in a New York minute.
Just another example of the Clintons feeling entitled and operating under their own set of rules. Also probably a good reason that they are going back and re-filing 10 years of tax returns due to errors and omissions that they just so happened to detect immediately following the release of the book Clinton Cash.