You say that you cannot recall a case that has attracted such publicity for so long without any developments, but the case of Amanda 'Milly' Dowler was reported for 6 months, from when she disappeared to when her body was found. As was the case of Elizabeth Smart, although this was in the US. She was taken from her room in 2002 and the case was covered for 9 months, until the day she was seen walking with the people who had taken her. In this case the media attention really helped as without it, it is unlikely she would've been found.
There are many other children, as you state, that deserve as much media attention, the problem is is that they are not 'newsworthy' enough. If you haven't noticed the media only portray certain types of children with certain family backgrounds. I'm sure not a lot of you will have heard of Hannah Williams, Daniel Nolan, Adam Morrell, Damien Nettles, to name a few cases that got barely any media attention.
With the case of Maddy it will continue to be reported until there is a conclusion, maybe not with as much coverage as now but there will continue to be reports. This is because people want to know what has happened to her and while there is still a need for this to be answered then the case will continue to be reported within the media.