Google has decided to drop Google due to lack of user interest. It doesn't have choice. what is the use of continuing a social network service which doesn't enjoy popularity among the users.
Google wave is too complicated for the average user - they never focused on making the platform open and easy on the average user It's a shame because their property could have significant applications to all people.
That's the basic reason Google Wave haven't got the adoption rate. And it's really sad to see a Google Product going away.