I would assume it was a way of keeping the audience somewhat in suspense. It's like any story with more than one plot happening at the same time - the author (or filmmaker) takes us back and forth between the two plots, the two groups of characters, until finally they've all arrived together.
The link below explains the choosing of the caskets in a very interesting way. Given that it's pretty obvious what's going to happen (two suitors will pick the wrong ones, then Bassanio, the one Portia desires, will pick the correct one), perhaps it was Shakespeare's intent to simply draw it out more for his audience's enjoyment.
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