Because they've paid a lot of money for him and he's on huge wages and a 4 year contract. Clearly they expect him to be around long term or why invest the thick end of £40+ million - especially where there's unlikley to be any sell on value.
Its also the case that RVP is, as things stand, obviously much the better player - to be expected considering his experience. Welbeck may surprise me and develop into a truly world class forward (and of course I hope he does) but he's got a long way to go if that's ever going to happen.
For the record I like the lad, and he's got a huge part to play - but in terms of being a regular he's up against two of the best forwards in the world, as well as a player who while probably not offering as much in general play but has the rare knack of scoring regular goals.
I therefore think that in a year or two, if he wants to play every week and be an England regular he may start to get itchy feet. Hopefully he develops and can force his way in on form and ability.