Par for the course in the hardware world, one of the reasons why I'm very happy my soldering iron has retired.
The probability of a chip being eol'd for <insert random reason here> is inversely proportional to the number of pin compatible replacements made by alternate sources.
And that makes perfect sense: If something is essentially guaranteed to sell well, competitors would produce it as well. If something is not selling well enough to produce compatible replacement, then it is likely marginally profitable to produce or not at all.
The probability of a chip being eol'd for <insert random reason here> is inversely proportional to the number of pin compatible replacements made by alternate sources.