Seeing HTC's initial success with its Sense skin on top of Android in their Hero phone, Motorola wanted to do something similar and this is how their Motoblur skin was born in their first Android phone - the Droid. The skin has been criticized quite a lot, because it wasn't as good as HTC Sense, and it was very distracting.
Even though their new version of Motoblur for Droid X was much improved, Motorola has decided to stop focusing on it going forward and instead focus on their phones more.
"With MOTOBLUR, we have found that being able to convey the value proposition around MOTOBLUR is not an easy thing to do in a 30-second ad spot. We have decided that we will focus on the value proposition of products and not MOTOBLUR as a brand name in its own right. MOTOBLUR continues to be important and I think you will see increased functionality in MOTOBLUR. This notion of push-Internet is going to be very important to us, but as a brand name, which we make matter in front of consumers as a brand name, I don't think that's going to be our focus going forward, but we see the experiences that we deliver is being relevant and differentiating us." Sanjay K. Jha, Co-CEO Motorola
Motorola and Google have been good partners ever since their great success with the Droid, on which they worked together to make, and since it's rumored that Google wants to make skins pointless with the design overhaul of Android 3.0, it's likely that Motorola has decided that they will use the untouched stock version of Android 3.0.
This would save them a lot of time and costs with the extra development they would put in the Motoblur skin, and it seems like a great decision, especially considering the skin hasn't been appreciated and their customers want fast updates, as well.