I mean, if I were going to plan a terrorist attack, I'd go all out. There wouldn't be a country left standing when I was done. I wouldn't kamikaze charge the Parliament building and hope for the best, because that's stupid and ineffective.
And perhaps that is the point. Stupid, ineffective, low-cost attacks which rile Western nations and put pressure on greater forces for intervention, leading to a land invasion of Islamic State territory, which can then be used to rally Muslims against the "Western Crusaders and their Zionist masters".