The number of tornadoes this year is way down and is the lowest since 2002. It is possible that once the adjusted reports are completed that the number of tornadoes is the lowest since 1988. May which is typically an active month for tornadoes averaging 295, had only 96 tornado reports. April was the most active month for tornadoes with 146, most of which came with one storms system early in the month.
Now this is all good news because along with the number of tornadoes that are down, the number of tornado deaths is down with only 3 reports so far this year.
If this tread continues, 2018 could have the lowest number of tornadoes on record, but keep in mind, we still have tropical season and the second severe weather season in the fall to go yet.
The main question is why no tornadoes. One thing that stands out is the PNA (Pacific/North American) pattern. A negative PNA pattern was in place for most of the spring which lead to warm and dry weather pattern across the eastern have of the country and a lack of the jet stream taking major dips into the Plains and East. During a positive PNA, greater trend toward storminess in the eastern half of the country and a greater chance for rich Gulf of Mexico moisture to be drawn north to help fuel the thunderstorms. We also saw blocking going which lead to cooler weather for the Northeast. So while I am being very simplistic in my answer, the basic answer is, the pattern for tornadoes has not been there this spring.
As a side note, 2002-2003 was a snowy winter for the many parts of the country, especially the Northeast.
Below are images from the SPC site showing the tornado trends so far this year.