Why? It depends on your needs. For me I don't have a 4k TV, or subscribe to Netflix, never got past 60% full on Tivo or needed more than 3 tuners and as highlighted in this thread, the lack of a scart out has caused some issues. A faster box would be nice, but that's not quite enough and Tivo has got quicker since the last update. Oh and the minor factor that VM are charging for the 'upgrade'. Look, and be fully informed before you leap is never bad advice.
There is a charge but when I enquired I was offered the new faster modem an upgraded from vivid 100 to 200 for £1.50 a month less than I was paying. I just paid £49.99 for the box and I am pleased that I did. I am lucky because nearly all of my wires are in the garage.
If you think I was paying over the odds to get the deal I can assure you I am not.