Because he had to. The collection _Mark Twain's Library of Humor_ had
been edited -- ie assembled -- a number of years before by William Dean
Howells and Charley Clark, an editor at the Hartford Courant. Lest you
think Howells performed this service out of the goodness of his heart, he
did it because his friend SLC hired him to. The project then got caught
up in the shift of affiliations following the break-up of James Osgood's
and Henry Houghton's publishing empire, which included _The Atlantic_.
The collection foundered until SLC, desperate to shore up the floundering
fortunes of C. L. Webster and Company, his publishing firm, decided to
run it himself. But Clark and Howells had gathered together some
copyrighted material, which SLC only realized at the last minute. Hence
his begging Belford for permission. Hope this clears up your question .