This has been discussed in several other recent threads in this forum. The developers have been busy this week putting up Tails 0.16 (with increased urgency owing to the TURKTRUST breach), so I'll try to help out by saying what I think they'd say with greater authority and accuracy than I can manage.

The Tails developers have stated that Whonix is indeed an interesting and promising project, but for various reasons, the Tails Project and Whonix Project are not likely to merge in the near future.

If you want to see their own words, try using the "search" tool above to look for threads mentioning "Whonix" and inside those threads, look for posts by "Tails".

The developers have been busy this week putting up Tails 0.16 (with increased urgency owing to the TURKTRUST breach)

FTR, Tails 0.16 release cycle was published more or less according to schedule. Our current workflow and infrastructure does not really allow hurrying things to release much less that a week after a Firefox ESR is out.

why tails developers dont unit efforts to create software like whonix?

The main current reason is about diverging design goals: Tails has a focus on not leaving traces on local media (see about and design), while Whonix has other valuable priorities.