The SPU was once a real player in Ukrainian politics, but it last had sets in the Verkhovna Rada (Ukraine's parliament) in 2006.
They were excluded from the Socialist International in 2011, but the reasons for this seem obscure. Pravda.ua writes about this event (in machine translation):
The council was chaired by SI President and Greek Prime Minister Georgy Papandreou and was dedicated to the new wave of democracy sparked by the "Spring of Nations" in North Africa. [...]
Among the decisions of this Council, taken under the influence of the "Arab Spring", was the removal of a number of parties from the ranks of the Socialist International for non-compliance of these parties' actions with the fundamental values and principles of the International.
Although the purge has primarily affected Arab countries, two parties in Eastern Europe have been expelled from the SI. These are the Bulgarian Party of the Left Social Democrats and the Socialist Party of Ukraine of Oleksandr Moroz.
That's not every illuminating as for reasons why the SPU was excluded then. Neither Moroz nor the SPU appear to have been involved in any scandals around that time, as far I could investigate. And there wasn't an alternative party admitted in the SI in SPU's place, as far as I can tell, either. (That's sometimes a reason why a party is excluded, e.g. it splinters in two groups and one manages to get the upper hand in these international fora.)
So, is there anything more known about why the SPU was excluded from SI?