I would say: because.
You can actually skip the genitive noun phrase "der Probleme" to get to the actual grammatical phenomenon. It is the rough equivalent of "one" as a single pronoun in English, like "I have seen one". Let me outline outline the declension pattern (in singular) of that pronoun:
[masc.] einer [nom.] einem [dat.] einen [acc.]
[fem.] eine [nom.] einer [dat.] eine [acc.]
[neut.] eines [nom.] einem [dat.] eines [acc.]
Note first that there is no proper genitive case when the pronoun is used alone. If the pronoun is accompanied by a genitive phrase, say with an definite article, then you can use
[gen.] eines der [masc.] einer der [fem.] eines der [neut.]
Note second that the endings of "ein-" are precisely like you would usually do with the indefinite article except for the nominative case. The underlying cause for appending the suffix, from the perspective of economical use of language, is probably to strengthen the case marking.