In fact supporting HTTP/2 often required radical changes to their internal implementation due to many factors (practically always required encrypted connections, capability to distinguish between HTTP/1.x and HTTP/2 connections on the same TCP port, binary representation of HTTP messages, message priority, compression of HTTP headers, use of streams also known as TCP/IP sub-connections and related flow-control, etc.) and so a few developers of those web servers opted for not supporting new HTTP/2 version (at least in the near future) also because of these main reasons: protocols HTTP/1.x would have been supported anyway by browsers for a very long time (maybe forever) so that there would be no incompatibility between clients and servers in next future; implementing HTTP/2 was considered a task of overwhelming complexity that could open the door to a whole new class of bugs that till 2015 did not exist and so it would have required notable investments in developing and testing the implementation of the new protocol; adding HTTP/2 support could always be done in future in case the efforts would be justified.
In fact supporting HTTP/2 often required radical changes to their internal implementation due to many factors (practically always required encrypted connections capability to distinguish between HTTP/1x and HTTP/2 connections on the same TCP port binary representation of HTTP messages message priority compression of HTTP headers use of streams also known as TCP/IP sub-connections and related flow-control etc) and so a few developers of those web servers opted for not supporting new HTTP/2 version (at least in the near future) also because of these main reasons protocols HTTP/1x would have been supported anyway by browsers for a very long time (maybe forever) so that there would be no incompatibility between clients and servers in next future; implementing HTTP/2 was considered a task of overwhelming complexity that could open the door to a whole new class of bugs that till 2015 did not exist and so it would have required notable investments in developing and testing the implementation of the new protocol; adding HTTP/2 support could always be done in future in case the efforts would be justified