My instinctive reaction was 'lack of'.

Hier geht es um die Abwesenheit von Personen. "Failure" bedeutet das Ausbleiben einer Sache oder eines Vorgangs, aber niemals die Abwesenheit von Personen. "lack of" würde implizieren, dass es überhaupt keine führenden Persönlichkeiten gibt. Aus dem Text geht aber hervor, dass es sie gibt, dass sie aber ausgewandert sind. Daher: absence

In those times, eminent proponents of democracy were sorely missing/missed: people like Carl Schurz, for example, who had left Germany in .....

I wouldn't translate "...machte sich bemerkbar" with "were drawn to attention".

At that time the lack of leading democratic personalities was making itself painfully obvious; people like Carl Schurz...
in order not to become.... (better: in order to avoid becoming)

One way of doing it

At the time leading democratic personalities like Carl Schurz, who had abandoned Germany after 1849 in order not to become a victim of the reactionary policies of most of the German states, were sorely missed.

Thanks heaps for helping :-)

You need to get the punctuation right :

At THAT time, (comma) leading democratic personalities like Carl Schurz, who had abandoned Germany after 1849 in order not to become a victim of the reactionary policies of most of the German states, were sorely missed.

not sure about "abandoned Germany".
MW has for abandon:
Main Entry: 1aban·don
Pronunciation: &-'ban-d&n
Function: transitive verb
Etymology: Middle English abandounen, from Anglo-French abanduner, from (mettre) a bandun to hand over, put in someone's control
1 a : to give up to the control or influence of another person or agent b : to give up with the intent of never again claiming a right or interest in
2 : to withdraw from often in the face of danger or encroachment
3 : to withdraw protection, support, or help from
4 : to give (oneself) over unrestrainedly
5 a : to cease from maintaining, practicing, or using b : to cease intending or attempting to perform

If at all, only meaning 2 can be meant in your context, but even then I think it is too strong. What's wrong with simply saying "who had left Germany..."?