he was trying to cut off every member of the previous legistlation council from joining the new one. unfortunately the next council was even more incompetent in the long run.

http://muse.jhu.edu/journals/fhs/summary/v025/25.4shapiro.html

This article argues that the main motive behind Robespierre's proposal of the self-denying ordinance was to block the future influence (and the political careers) of the 'triumvirate' - Barnave, Lameth, Duport - leaders of the moderate and pro-constitutional-monarchy group, the Feuillants. These Robespierre suspected of being in cahoots with Louis XVI (a not-unfounded suspicion; Barnave later became a minister), and possible obstacles to its abolition. Robespierre and many other radicals of the time, such as Desmoulins were aware that he was impeding his own career through the self-denying ordinance - it was celebrated as an act of self-sacrifice.

Different members of the Constituent Assembly had different reasons for supporting Robespierre's proposal - whether because they agreed with him (the left), or because they hoped it would undermine the new constitution (the right, or because they didn't really want to be re-elected anyway (bearing in mind, these were still the same deputies who were elected for the specific purpose of convening the Estates-General and probably did not have a revolution in mind at the beginning), or because they felt they could not oppose a proposal that seemed to so obviously stem from selfless virtue.