"wouldn't it make sense to determine if a initiative is unconstitutional or not before they attempt to place it on the ballot? "

Yes, this should be done but it is up to the drafters and proponents of an intiative to determine this before they present it to the public for signatures. A lot of the intiatives are thrown out because the proponents do not do their homework or are unwilling to pay an attorney to do the reasearch to properly draft the language to begin with.
A lot of intiatives are drafted by layman that do not know the precedents set by case law prior to collecting signatures for to get the intiaative on the ballot.

Generally, until a new law is litigated and an opinion rendered by a court, the effect of the law is somewhat uncertain.
I may be wrong about the above, but this is my understanding about one problem with the intiative process in general.