Why are they rebooting/reviving The Matrix franchise?

There's an answer in the article:

While some at Warners consider the title among the studio's sacrosanct properties, like Casablanca, others see a need to redevelop it in an environment where studios are desperately looking for ways to monetize their libraries and branded IP is hard to come by.

Basically, they are looking to make more money of an already successful franchise to which they own the rights.

But Warner Bros. sees a model in what Disney and Lucasfilm have done with Star Wars, exploring the hidden corners of the universe with movies such as Rogue One: A Star Wars Story...Perhaps a young Morpheus movie could come out of the exploration, as an example.

It could be that they explore different characters, but there's no official word on what it is going to be, but it is unlikely to feature Keanu Reeves:

While promoting John Wick: Chapter 2, Reeves said he would be open to returning for another installment of the franchise if the Wachowskis were involved. "They would have to write it and direct it. And then we'd see what the story is, but yeah, I dunno, that'd be weird, but why not?" he told Yahoo Movies. However, it is likely that Warners will look elsewhere to attract an A-list director and star.