You need "because of". "a build-up of excess stocks held by personal computer markets" is a noun phrase and should be preceded by a preposition (because of). "because" (it's a conjunction), on the other hand, can be followed only by a clause.
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AnonymousSubject is "a build-up of excess stocks" & Verb is "held".No. '[that] is held by personal computer markets' is a relative clause modifying 'stocks'.