Based solely on news reports, there's been a rising trend of citizens of countries in Western Europe and North America joining the terrorist organization calling itself "Islamic State". This seems to me to be in contrast with older movements failures in these areas, such as Al Qaeda, who I can't recall doing nearly as much recruiting in these places. What's causing this difference? Why is Daesh so successful relative to other organizations?
The Internet and Propaganda
IS is using social networks and superbly produced magazines while Al-Qaida stuck to video messages.
Boring, lengthy, intellectually challenging political speeches are more effective if they are accompanied by constant news from inside the organization. Chit-chat about how great the weather is in Syria and Iraq, while useless for radicalization purposes, creates a personal bond - which is great for persuading.
Lifestyle and Religion
Modern life is confusing; A clear and easy distinction between good people and evil kuffars (with no grey tones - Shia are as bad as Jews) makes that much easier.
It also helps if all bad things happening to you, are considered as a result of the actions from the other site (And even if that assertion is true, forgetting to considering their reasons).
Special clothes, beard styles, hymns and a lifestyle separate from the mainstream society further the divide and create a group identity. The Verfassungschutz NRW (Office for the Protection of the Constitution - State of North Rhein-Westphalia) actually calls Salafism a form of counter- and protest-culture.
A large(r than their representation in the whole Muslim community) portion of the terrorist are converters. They faced some personal crisis which was solved with their new faith in Islam - they attribute a personal value to their faith which gets attacked and so fear that they would return to their worse situation.
Anti-Muslim sentiment and War
It's not terribly hard to spin the NYPD bugging Mosques, PEGIDA, general anti-muslim sentiment and drone strikes on weddings into attacks on Muslims in general - not just on terrorists - which can incite anger.
The special brand of Sunnism which is the terrorists favorite choice is Salafism jihadism. It also is the brand pushed by Saudi Arabia (Often called Wahabism in English texts. There's a discussion whether those are actually equal - the asserted differences don't matter for this answer). Wikipedia cites a lower bound of $87 Billion from 1987 to 2007. That's a lot of textbooks, imams and mosques spreading questionable content.
There are some AFAIK rumours that Saudi-Arabia & Qatar initially financed Daesh directly. There is no doubt that money came from those countries but I'm not aware of any proofs the governments are involved directly. Which would leave that money coming from the wealthy
Oil monopoly is major source of Islamic State power
Smuggling to Iran via Kurdish areas raised $1 mln/daily
Long before it looted the Iraqi central bank in Mosul of $429 million, the Islamic State in Iraq and the Levant/Syria (ISIS) was well-funded in its quest to establish a Shariah caliphate, thanks to a raging criminal enterprise of extortion, bank robbery and petty theft, as well as donations from well-heeled sponsors throughout the Arab world, according to experts. ...
But much of their funding comes from what they consider "taxes and voluntary contributions," but others likely consider extortion and protection money, Lund said.
- Misuse of Zakat
Zikat, being one of the five pillars of Islam, makes sure there is always much money for charity. Now if you're a terrorist you won't have that much of a problem not investing that money in blankets and foods (Which Hamas does - giving $X to Hamas does NOT mean that $X will end in the hands of the Al-Quassam brigade. Though it's naive to believe that none will), but in rockets or propaganda material (After all it's for the greater good).
If that money is blocked wrongly that only increases the perception of anti-Muslim sentiment
A lot is due to anti-Assad propaganda. A lot of people joined ISIS to overthrow "tyrant Assad" when it was still supported by the West as part of Syrian opposition.