Because their priorities have changed and they're now entirely focused on the urban / luxury market. They used the excuse of emissions and safety as their way out. They never intended to seriously redevelop Defender. They've spared no expense on all other models and they would have done the same for Defender by now if they were serious.
It's why they came out with an fj cruiser type nostalgia design for the urban funkster market. That got rejected. Now they have no idea what to do. None of their designers, managers or marketing boffins want to design a work truck because it's no longer their market priority. Which is death to the Land Rover brand in the long run IMO. It's been obvious for years. Look at their priorities.
You only have to walk into a JLR showroom now and it's very stark. Without any Defenders on the floor and with the attitude of sales staff it's clear. Land Rover is a luxury brand only now. That's why they have no idea how to re-design a heavy duty pragmatic working vehicle like Defender. Which is why there is no confirmed new design, why the arbitrary date keeps getting pushed back ...and why Defender is finished.

