Three things:
Land Rover says the Defender was not going to meet new Euro emissions standards, and perhaps the cost of reverse engineering or upgrading it was going to be so high they decided to invest that money in trying to start a new chapter with the Defender's replacement.
I don't know why they couldn't just source a different engine, but they might have decided this is the right time to also address all the other issues and quirks that Defender owners either love or endure, but which make it a non-starter for new generations of drivers who expect a higher base level of technology.
Secondly, there could be a financial plan at work. Look at the annual sales figures for Defenders in Europe on this page: Land Rover Defender European sales figures
They're small and tapering off to abysmal. Rather than trickle along like that and reach a point where people are still thinking about the classic defender when the new one is launched, they may want to build some nostalgia and longing for a year or two, run an 'It's coming...' marketing campaign, and launch with an explosion of sales to new customers who would never have bought the current Defender but are willing to buy a modern LR that trades on its legend.
Land Rover was bought in 2008. The current owners could be losing money already on every LR being sold because of the low volume.

