While you may see this as a recent change in news reporting, it is neither restricted to the press nor a new development, though the nature of online media consumption may have promoted it.
There is no requirement that paragraphs be of a certain length or contain a certain number of sentences. Those considerations, as well as whether or not to organize ideas into paragraphs in the first place, depend on the medium. Single-sentence paragraphs can be found in works written centuries ago, and in literature, political writing, technical writing, and every other sort.
I would accept that news may be particularly susceptible to the phenomenon. In expository writing as taught in schools, information is presented piece by piece, supporting a thesis and leading up to a conclusion. News reporting, on the other hand, organizes information by relative newsworthiness, with the most striking information presented in the lede— no thesis, no conclusion. In a story written in the inverted pyramid style, one could cut off the story at the end of most paragraphs and remain cohesive. Information of subordinate newsworthiness is pushed into later paragraphs, and if only one sentence of information is worthy to sit at a certain place, so be it.
Newspaper articles (and their online equivalents) are not essays, and have not been for a very long time. If you look up newspapers from the mid-20th century, you can see that articles opening with one- or two-sentence paragraphs is hardly new.
These materials being presented online is another factor. Reading from a computer screen is slower and more fatiguing than reading from a printed page: long line lengths on screen that make it difficult to follow multiple lines of text; the flicker of CRT monitors and the insomnia-promoting blue of LED backlights increase eye fatigue; poor screen resolution makes characters more difficult for the brain to comprehend.
Therefore, various guidelines for writing on the web emphasize brevity (short sentences and paragraphs) and chunking (organizing material in to small groups with a headline, for easy scanning while scrolling). It is easier to keep your place in text if there are only three vertical lines to scan, not ten. And we are more likely to remain on a website which is easier to read. Online forum participants perpetually complain about the failure of new members to break up their text, and the expression TL;DR is lobbed against responses such as the one I am composing right now.