资料：Native ads - or ads that take on the look and feel of the content surrounding them - are taking over digital advertising. By 2021, native display ad revenue in the US, which includes native in-feed ads on publisher properties and social platforms, will make up 74% of total US display ad revenue, up from a 56% share in 2016, according to new BI Intelligence estimates based off historical data from the Interactive Advertising Bureau （IA B） and PwC, as well as IHS. The rapid uptick in native"s share of display ad revenue can largely be attributed to the dominance of social platforms like Facebook and Twitter — which were early champions of native and rely almost entirely on native formats — as well as the introduction of new programmatic technologies that are making it easier for publishers and advertisers to scale native campaigns. In a new report from BI Intelligence, we breaks out native ads into three categories: social native, native-style display, and sponsored content （also referred to as premium native）. We provide（一审未录入） forecasts for how revenues from these formats will grow over the next five years and looks at what factors, in particular, are driving up spending on each of these ad units. As a note, because revenues from these three types of native content can overlap, we do not provide an overall native forecast. Finally, we lay out some of the challenges that face properties that rely on native ads, namely ad frequency and Scalability issues. Here are some key takeaways from the report: Native-display ads, including social native and native ads in-feed on publisher websites, will make up the bulk of native ad revenue from 2016-2021. Native display ad revenue in the US will rise at a five-year compound annual growth rate （CAGR） of 17% during this time period to eclipse $36 billion. The rise of native video ads, particularly on social platforms, will be one of the main drivers of this growth. According to the passage, what is NOT the reason for the rapid uptick in native’s share of display ad revenue?