I think it is because of the 'Into Darkness' and a story featuring terrorism, revenge, murder, suicide bombings, grim deaths etc instead of a more 'fun' adventure.
Although I loved STID, it seems too mature for my older grandchildren (7 and 5). Hopefully, the next movie will be more family-friendly.
I am concerned that marketing was too dark for many families in US. YMMV.
“When all Americans are treated as equal, no matter who they are or whom they love, we are all more free.” -Pres. Obama
"A great democracy does not make it harder to vote than to buy an assault weapon." -Pres. Clinton