Just reading these two wiki pages, http://en.wikipedia.org/wiki/Nuclear_weapons_and_Ukraine, and http://en.wikipedia.org/wiki/Budapest_Memorandum_on_Security_Assurances , And it didn't say much about the motivation Ukraine had for getting rid of the nuclear weapons except for the ambiguous and unsourced "Certain Ukrainian leaders are angered by the West because Western Europe and America wanted them to rid their nuclear weapons."
This New York Review of Book blog by a historian of nuclear technology physicist answers some of the rationale behind the Budapest Memorandum. In addition to economic assistance, there was enormous international and domestic pressure for Ukraine to disarm its nuclear stockpile. Ukraine was the last holdout of the former Soviet Republics with nuclear weapons, which in addition to its size, made retention of them a difficult proposition. Domestically, Ukraine's parliament was highly fractious and the budget was a significant source of tension with the executive. Getting rid of the stockpile relieved this tension, especially since maintaining these weapons against the tide of international opinion would have been near impossible for Ukraine's constrained economy.