This just adds burden to programming language and library authors. Sure I could almost certainly support Python 2.7 and Python < 3.6 in all my libraries, but it introduces more code and shims I've got to maintain. Given I'm not being paid for this work and do it for fun, there's no compelling reason to support the small subset of users that cannot or will not update.

I imagine variations of the above apply to nokogiri and some amount of Ruby core developers.