About the Fed Cup it was probably the same reason why Vika gave up playing doubles with Maria Kirilenko in 2011. Basically she wanted to concentrate to singles matches in tournaments. Only few top 10 players like Serena Williams with Venus Williams (in Grand Slam tournaments) and Sara Errani with Roberta Vinci (in most of the tournaments) play both singles and doubles. Even Maria Kirilenko has decided to stop playing doubles in this year. http://www.tennisworldusa.org/WTA---...icolo8414.html
Besides Belarus would need larger roster of players in top 50 to be successful in Fed Cup. For instance the Czechs have one of best doubles team ( Andrea Hlaváčková & Lucie Hradecká) and 3 other top 30 players (Klara Zakopalova, Lucie Šafářová & Petra Kvitova). It is much easier to build a team like that when it is not only based on how well Petra Kvitova plays in Fed Cup.