Why no removable faceplates on 360 slim?

Little interest in them meant low sales
When the Xbox 360 was designed, removable and replaceable faceplates for cell phones were a big deal; and apparently Microsoft thought it would be a great idea to give owners the ability to change the faceplate on their Xbox 360 in an effort to allow the customization of the Xbox 360 hardware itself.
When the Xbox 360 Slim (or Xbox 360 S, if you like) was in the design stage, it was decided that Microsoft would not offer the removable faceplate option, since it never sold well the first time around; and Microsoft has had more success selling limited or special edition versions of the consoles that are customized, such as the Halo Reach and Call of Duty: Modern Warfare 2 console offerings. Whispers suggest that we can expect more limited edition or special edition customized console offerings in the future.
We think that one of the biggest factors in the faceplates lack of popularity was that the number and types of faceplates offered were not that compelling. In addition, many of the best or most successful faceplate offerings were also very limited edition or very hard to get due to small production runs. One source admitted to us that perhaps it could have been done better, but by the time that was realized it was already the end of the road for the faceplate feature.
One thing that the discontinuation of the faceplates has done is driven up the market for those who collect the Xbox 360 faceplates, with very limited edition faceplates fetching some very high prices. The less exclusive but popular offerings are also in demand at prices that some may find less than reasonable. Still, it does remain one of the best ways to customize your non-slim model.