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From Wikipedia, I gathered the following data (for the European Union):
2015: 17 attacks by Islamists with 150 deaths.
2016: 10 attacks by Islamists with 135 deaths.
2017: 10 attacks by Islamists with 62 deaths.
2018: 4 attacks by Islamists with 9 deaths.
What is the reason for this massive reduction in Islamist terror attacks? Is it possibly related to improved intelligence and ability to prevent attacks? Is it related to the fall of ISIS? Or is 2018 just an outlier?
The first thing to note given your question, in which you cite the number of deaths from terror attacks as well as the number of terrorist attacks by Jihadists, is that each of these years 2015 and 2016 had a single event that accounted for the majority of your cited deaths in Europe for each year. The November 15 Paris Attack in 2015 and the Nice Truck Attack in 2016, which accounted for 137 and 87 deaths respectively. In your gauging of what constitutes a spike and decline of terror attacks you'll need to decide which metrics are the most helpful for interpreting this data.
A metric perhaps less susceptible to being skewed by one large attack or several small attacks would be the number of actual suspects arrested. Below you can see a graph for number of suspects arrested over time 2013-2017 (source)
As you stated there was a decline in 2018. Though the report from this source for this year will not be released for some time, it appears as though the decline is significant.
While it may seem logical to tie together the European migrant crisis to this, in sources I've been able to find the perpetrators of the worst attacks were European nationals who had been radicalized, and not radicals who snuck in with refugees. Though it would be unsurprising if inflamed tensions as a result of the migrant crisis aided in the radicalization of some attackers.
The most logical deduction explaining the rise and fall of Jihadist terror attacks in Europe during this time frame follows the rise and fall of ISIS, which claims responsibility for most of the attacks during the time frame of 2014-2017, and was most prominent as an entity during this time frame.
EUROPOL (The European Union Agency for Law Enforcement Cooperation) is the EU agency which combats these threats, and collects and disseminates info surrounding them. A summary of their observed trends as of 2018 can be found here.
