Because I think atheism at this point is far from any roots in humanism and is represented by a bunch of sneering geeks like Hitchens and Dawkins. Russell wrote:
Dogma demands authority, rather than intelligent thought, as the source
of opinion; it requires persecution of heretics and hostility to
unbelievers; it asks of its disciples that they should inhibit natural
kindness in favor of systematic hatred.
A good part of modern atheists could easily qualify for the above being applied in reverse, notwithstanding their disingenuous references to the atrocities of the [leaders of the] faithful.
Russell also wrote:
Three passions, simple but overwhelmingly strong, have governed my life:
the longing for love, the search for knowledge, and unbearable pity for
the suffering of mankind.
Russell was a mathematician who nonetheless felt no need for a calculus to substitute (in the quote above) for "natural kindness". Today, I am afraid, atheism is a product of an arithmetic style that would be (in its rhetoric) difficult to differentiate from the [non- religious-fanatic] right in identifying such emotions ("unbearable pity") derisively as "bleeding heart", etc.
--ravi