Because Messenger 8.0 Beta is out for Hotmail users that have signed up to be invited (however oxymoronic it sounds), and it seems that Messenger Plus isn't compatible with that version. Ah, well, while we wait for Patchou to come up with an update, I might as well go into detail about what Messenger Beta is.
To me the only drastic improvement over Version 7.5 was implementing Yahoo Messenger's computer calling feature (and it also is free). However, the style in which it was made is apparently an attempt at a futuristic interface, but now the only areas in which you can change the colour in the conversation window are the writing tabs, header, and Send button. The new activity icon omits the clock and no-entry symbols that indicate whether a contact is away or busy and instead is coloured light green. A deep green indicates a person that's online and active, and silver indicates that the contact is either online or has blocked you. Blocked icons are just small sprites.
Still, if you wish to sign up for Messenger 8.0 Beta and have a Hotmail account, you can do so here.