As if the worst foe of the Islamic state right now is the Russian Federation .. very suspicious especially when many embassies warned their citizens to leave Russia just a week ago because they had Intel about major terror attacks coming to Russia.
The Islamic State has plenty of reasons to hate Russia, such as their continued cooperation with the Assad government and Iran, the Chechen Wars, their military campaigns in Afghanistan... just to name a few. You don't need to be "the worst foe" to be targeted by ISIS, as many of their attacks have shown.
When it comes to foes Israel, US, France, UK, Germany, Netherlands; KSA, UAE should top the list.
unless someone else is pulling the strings and using them to advance their geopolitical agendas. terror attacks abroad have almost no impact on rallying support in the regions they want control over.
I imagine those other targets are in much better positions to prevent such attacks right now. Russia is kind of busy elsewhere, so maybe they were seen as a better target with a higher chance of pulling off an attack.