But I don't quite get it, can you rewrite the sentence?
Afghanistan has no formal party or primary system, hence the large field.
Afghanistan has no formal party or primary system, that's the reason for there being so many candidates.
In a party system, candidates of a particular party compete for one spot on the ballot. This is usually determined by a primary election. At the end, there is only one candidate per party on the ballot. That eliminates a lot of candidates.