Stability of the Government
The UNP and its allies secured 106 seats at general election 2015 and the SLFP and its allies secured 95 seats. In the history of Parliamentary elections under the new Constitution only in two instances was a clear majority of 113 seats secured by the winning party. That was in 1989 when the UNP secured 125 seats amidst the JVP insurrection and in 2010 where the UPFA secured 144 seats soon after winning the war. In all other instances there were stable governments with the support of a section of the Opposition either supporting from the Opposition or joining the Government.
If the Parliament was dissolved immediately after the presidential election in January 2015, the UNP would probably have secured sufficient majority in the Parliament. Also had the President dissolved the Parliament earlier, it would have been advantageous to the UNP. The President himself admitted this at the speech addressing the nation delivered soon before the general election 2015.
Strategic move
If Rajapaksa continued to be the Chairman of the SLFP, there would be an exodus of MPs from the SLFP to the UNP soon after the presidential election.
A few members left the SLFP and joined the UNP just before the general election after the SLFP gave nominations to Rajapaksa. In that case the battle-line was clear and the UNP would have been strengthened as a result.
However, this did not happen since the leadership of the SLFP was handed over to the President.

If the Parliament was dissolved immediately after the presidential election in January 2015, the UNP would probably have secured sufficient majority in the Parliament. Also had the President dissolved the Parliament earlier, it would have been advantageous to the UNP. The President himself admitted this at the speech addressing the nation delivered soon before the general election 2015.
Strategic move

If Rajapaksa continued to be the Chairman of the SLFP, there would be an exodus of MPs from the SLFP to the UNP soon after the presidential election.
A few members left the SLFP and joined the UNP just before the general election after the SLFP gave nominations to Rajapaksa. In that case the battle-line was clear and the UNP would have been strengthened as a result.
However, this did not happen since the leadership of the SLFP was handed over to the President.