iOS is seeing attacks decline
According to the PDF, with the authors claiming the reason is because Android attacks are profitable and iOS attacks are not. They say it has "dropped below meaningful percentage points" and thus don't even list the number of iOS attacks. I wish they'd have expanded on this, because it is interesting.
Obviously iOS is a minority of smartphones - a bit less than 15% worldwide by sales - but since they cost significantly more on average, the average iOS user comes from a richer country and is richer. Presumably a better target for malware, right? macOS has an even smaller percentage of PCs than iOS does of smartphones, and a lesser degree of cost difference from Windows PCs, but attacks there are rising. Why the difference?
Maybe it is because when a working attack on iOS is found, Apple can and will patch it very quickly, and it is applied quite quickly and widely, meaning that working attack is really only useful for targeted attacks (i.e. I want to break into Trump's Twitter phone) rather than trying to infect many people. A targeted attack can stay under Apple's radar and keep the exploit secret, but once it is released very widely it will become known and be fixed almost immediately.
While Google probably responds as quickly with Android patches, those fixes will never reach a majority of phones in the wild at the time the fix is made, so a working Android exploit may be thousands of times more effective due to its far longer useful life. Even if the average Android user has less money (worldwide, most are buying phones for $100 and under) the fact the exploit will be able to work for years means it will easily outearn iOS exploits that would have mere days to earn before they're shut down.
On PCs, Windows and Mac are probably about equal in how quickly fixes are developed and applied by end users. The fact most Mac users don't have AV software should mean they're easier to infect given a working exploit and therefore profitable enough to attack despite having something like 5% market share worldwide.