Because Congress (I) is in power. When Shiv Sena was in power, they named one art center as Prabodhankar Thakrey Bhavan. If it would have been BJP, they would have named it Shyamaprasad Mukherjee Bridge.
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Original name given to this project was Veer Sawarkar Bandra Worli Sea Bridge. It is only at last moment that the name is changed to Rajiv as inaugurated by UPA chairperson Sonia Gandhi. Anyways, name should not matter if it continues to serve the purpose it is built for.