The People’s Democratic Party (PDP), which Mark belongs to, has lost its majority status in the Senate.

Senate President, David Mark will no longer be the occupant of his seat, come May 29, 2015.

This is because the People’s Democratic Party (PDP), which Mark belongs to, has lost its majority status in the Senate.

The All Progressives Congress (APC) won 64 out of 109 Senatorial seats during the March 28 and 29 National Assembly elections and as such will now produce the next President of the upper house.

Political analysts say that the next Senate President could’ve come from the South-East but for the fact that the people in that zone voted overwhelmingly for the PDP hence cutting themselves off from the position.

The contest for leadership of the Senate is now said to be between former governor of Kwara State, Bukola Saraki, former governor of Benue State and Senate Minority Leader, George Akume, and Barnabas Gemade, a former National Chairman of Chairman of the PDP.

However, it is said to be unlikely that either Gemade or Akume would be chosen due to the fact that they are both from Benue State, which has occupied the position for eight years through David Mark.

Thus, this leaves Senator Saraki as the strongest contender for the position of Senate President.

Saraki will begin his second term in the Senate on May 29.

http://oyiidoma..com/2015/04/who-will-be-nigerias-next-senate.html

It would av been an easterner, but for the fact that there is no APC senator from that region.
Maybe from the south-south, but the only elected APC senator so far is a first timer from Edo north senatorial district.
Dat leaves the likes of Saraki, Gemade and Akume as favourites especially Saraki, because of the reason stated by @OP.

David mark as floor member. Lol.