Why was Jonathan Swift accused of misanthropy?

Not least because of "A Modest Proposal," the satirical essay in which he suggested that poor Irish should sell their surplus children to the rich for food:

https://en.wikipedia.org/wiki/Jonathan_Swift

Because of his writing.

Swift's supposed misanthrophy is epitomized in his book 'A Modest Proposal', in which he "suggested" the Irish unburden themselves of their numerous children — and break the cycle of poverty in the process — by selling them to the rich as food!

Many people however believe Swift simply had a darkly humorous outlook towards mankind.

Swift’s reputation certainly suffered at the hands of readers without a sense of humour. There is probably something in the charge of misanthropy: he found it difficult to feel goodwill towards the human race in general, though he maintained friendly relationships with men like Pope, Gray, Delaney and others, and he was certainly charitable to the poor.

As he said of himself in a letter to Pope: “I have ever hated all nations, professions and communities, and all my love is towards individuals .... Principally I hate and detest that animal called Man; although I heartily love John, Peter, Thomas, and so forth.” This strikes me as a more likeable attitude than that of most of our modern politicians, who profess undying love for the human race or the country but obviously can’t stand most of the voters.

https://www.sheilaomalley.com/?p=44615

For nearly all intents and purposes, Swift was, by mere definition, a misanthrope. Although his work “A Modest Proposal” is often cited, a more accurate and exhaustive account of his position concerning human nature is to be found in “Gulliver’s Travels.” Each book (I-IV) contains a satirical perspective concerning some degree of human nature, whether it be class systems, perverted learning, vanity, and so forth. A quick reading will no doubt conclude in one considering him an outright misanthrope. Ultimately, however, Swift has designed another tub which many are caught chasing (See “A Tale of a Tub”). In outlining his perspective on human nature, which appears obviously bleak, he provides us with a clear look at our behavior, and by discussing its pitfall; he furnishes us with the ability to improve our condition. Viewed in this light, Swift is a misanthrope, but one who cares deeply for his fellow man and wishes only for their improvement.

Source: david-reno.blogspot.com/2007/05/jonathan-swift-mightiest-of.html no longer exists
"Gulliver's Travels": http://www.gutenberg.org/etext/829
"A Tale of a Tub": http://www.gutenberg.org/etext/4737