Apart from the discussions in the comments about what constitutes an extreme-left party and what does not, I think this is a very complex question.
As usual the right-left separation, which is very popular in the media, is, to say the least, a very unsatisfactory one. It depends on the culture of each country, and depends on time. What was seen as left 100 years ago, might be right today, or the other way around.
In many countries, economic-liberals (damn you English, for misusing that word! ;-)) often constitute the centre-right, when conservative take the slightly more right spot (up to extreme-right's door). But economically, they often agree. So why should one be more centre than the other one?
But still, let's roll with it for now.
Where are they?
Well, Syriza took the power in Greece as you mentioned in your question. In Spain, you can look at Podemos, it is a political party which was created two years ago and reached the 3rd spot in the last general election breaking the bi-partism that was in Spain since the change to democracy. Some expect them to get better results in the upcoming re-election.
In Germany, Die Linke is a rather recent party, and in the 2014 European election, they received 7.5 % of the votes, ahead of the right populists Alternative für Deutschland.
In France, the 2002 and 2007 presidential elections showed a spread of far-left parties. So the Front de Gauche was created to present a single candidate for the "left of the left" in the 2012 election. It was partly successful in that, as one candidate was seen as the leader for the extreme left, but two other candidates were also present. Cumulated, they fell short of 13% (11% for the Front de Gauche alone).
And I could go on, but I think it is clear that extreme left parties are there and more or less successful. It should be noted that some left-wing parties tend to take more radical views, moving themselves more to the extreme, like ecologists both in France and Germany.
Why aren't they more successful?
Still, in the time of crisis, people tend to reject ruling parties and favour the extremes. But apart from the Southern countries, we mostly see a rise of the extreme right in Europe. Why that?
History. I won't cross that bridge to say that current extreme-right and extreme-left parties are copies of the 1930 parties, but there is a certain inheritance. 1930's extreme-right parties were actually more successful in Europe, as they ruled at least three large western-European countries. However after the second world war, and, in fine, since 1976, few countries were actually ruled by extreme-right parties. One of the reason is that the effective ruling of those parties from the 1930s were perceived as harmful in the post-war Europe. Nazis? But time passed, and the filter of history faded the colour of the time. On the other hand, until 1991, half of Europe was under control of a communist party. It is still very fresh in memory.
Economic paradigm. The extreme-left often stem from the conflict of classes. So the working class oppose the bourgeois, asking for a better re-distribution of riches. To keep it simple. That is an effective philosophy in the heavy-manned industries. A lot of people sharing similar work for low wage: they are numerous and all share the same incentive. They can effectively shut down the industry. Clothing industry, metal industry, mines, etc. were often stronghold of radical left. Since the 1970s, the economic paradigm has shifted from secondary to tertiary. Many people work in smaller companies of services, or the larger companies are more separated: the finance department, the engineering department, the service department, etc. So smaller groups and with diverging interests.
Perceived weakness. Our societies are under real and perceived threats. I will oversimplify, but they are related to the relationship of Europe's societies with Islam. Some people fear a cultural invasion by migrants, whether real or not, a fear is not always rational. However the threat of radical jihads is very real. The extreme-left typical approach is based on the social: no Islam isn't a problem, poverty is. Whereas the extreme-right is more on the line: yes they are a problem, even the pacific one, and we should protect ourselves. Strong nationalistic state appears as a better answer than equality and social justice.
And I think one can add a few other reasons, some related to those three, some independent, but I think those are the majors ones, and allow to answer your question without having too long an answer.