The main goal of Russia Putin is an instability in Ukraine. To show his own inner populace that "Don't even think of anti-gov protests. It ends up pretty bad. Look at Ukraine. You want like that?". Also, welthy and succesful Ukraine - is another major threat for Putin. For russians are convinced that 'democracy and capitalism is not for them'. Since ukranians are pretty much similar to russians, they\ll accept it as a possible path for them and, yet again, might decide to have their own Maidan.
So postponed presidential elections in Ukraine is a part of supporting instability. The **** is that a lot of candidates (especially Yulia Timoshenko) are also not interested in presidental elections in May.