The reason for this lies with the fact that 21st century teenagers have more sources of information and entertainment, aside from books. From their TVs, to their laptops, to their tablets and smartphones, it is easier for them to get information from these sources than good old books.
And because these sources can also be more interactive and fun, they no longer have a reason to read. They can simply watch a documentary, read a condensed summary, or even watch an "educational movie" as easier alternatives to reading.
Unlike teenagers who went through their younger years without 24/7 cable TV, streaming, smartphones, and the Internet, books were their only source of information. So, it only makes sense that they would treasure books more than today's teenagers. Looking at it from this perspective, it is easier to understand why teenagers no longer like to read books.