One main reason is because Rooney is forced to deal with the opposing defence without any support. It is clear that he hates playing as a lone striker, and he needs a strike partner he can play off. The reason he is forced to deal with the defence alone is partly because we do not have anyone capable of pushing up from midfield to help him out, and also because our wingers can not play the roles of both striker and winger.
As much as we hate to admit it, we are much worse off without Ronaldo. As i said in the paragraph above, Rooney becomes frustrated because of the lack of support coming from the wings and the centre mids. Ronaldo was a rare talent who could fulfill his role of winger whilst being allowed to roam freely and assume the role as almost another striker, making life much easier for Rooney, and giving our attack another dimension.
Having Rooney and Ronaldo breaking down the defence allowed Park Ji-Sung to do what he does best: run all day long and win the ball by hassling the opposing defenders and midfielders. However, now that Ronaldo has gone and Valencia has replaced him on the right side, it is much harder for Park to continue to focus purely on applying pressure on the ball-carrier, as he is forced to play in a flat 5 midfield rather than as an attacking winger.
Obviously, there will be occasions where only one holding midfielder will not be enough to shut down the midfield, such as if we come up against Barcelona for example, but for the most part, we need to send the message to our opposition that we are there to attack, not sit back and hope for a point. In particular, i feel that we should never use this formation at Old Trafford if we are going to make teams fear playing there.