Why did Isis attack Moscow?

Even Putin must realise deep down that the terrorist attack on the concert hall outside Moscow which killed more than 130 people was not inspired or authorised or carried out by Ukraine. This was pure Isis, brutal, ferocious, unforgiving, single-minded and fanatically planned. Why a concert hall in Moscow? Because Russian Spetsnaz special forces have been ruthless against Isis in Syria and this is their revenge. I have no doubt they timed the attack to coincide with Putin's reelection for another six years to demonstrate to him that as a militant Islamic organisation they can strike when they want and where they want. The Isis faction involved appears to be the Islamic State - Khorasan Province (Isis-K) which operates from Afghanistan. Their remit, certainly in Moscow, was to kill as many civilians as possible over a short period before trying to make a getaway. This is the same terrorist group which caused mayhem when American troops were trying to evacuate from Kabul, killing at least 150 Afghans and 13 US service personnel. Putin claims to have arrested the four gunmen involved, so presumably, if they are the real shooters, this will help to lift the finger of suspicion from Kyiv. Isis-K will remain one of the most deadly terrorist groups in the world, now that what is left of al-Qaeda is in hiding and no longer operational. Not at the moment, anyway. Their new leader is, interestingly, based in Iran and no doubt being looked after by the ayatollahs. Without a shadow of doubt, Putin and the Kremlin were caught napping with this latest terrorist atrocity, despite a warning from the US that an attack of this kind was imminent. Putin laughed at the US and has now paid the price. Or at least more than 130 Russian people and 100 others wounded in the terror strike, have paid the price for Putin's complacency.