Why are there state and local issues on a ballot when Americans go to vote for their next president? For example, Hawaii has this measure regarding jury trials in civil cases on the ballot for November 8, 2016.
Amendment of any constitution, state or federal, should be decided by a plebiscite.
A plebiscite (referendum) and elections cost a lot of tax-payer's money. It will be decided by state legislature when a state plebiscite will be held. Depending on how important and crucial an issue is, it could be held on presidential or mid-term election day or on a separate day. I don't think increasing the threshold from $8,500 to $10,000 for jury trial is important enough to warrant a separate day for voting.
Also, elections for 34 senators of the 100 seats in the Senate and all representatives from all 435 congressional districts will be held on the same day. Why not a state referendum for a small state issue?
In addition to Rathony's very valid and excellent answer, an additional reason is the voting rate.
Far more people are likely to come to the polls to vote for President, than to vote for a minor referendum issue like this.
As such, scheduling the referendum for a separate day would likely significantly reduce the rate of participation, which has a whole host of implications, some material and some moral/philosophical.
Yet another additional consideration to avoid scheduling on a separate day is costs, but NOT only the cost to the state to run a referendum but 2 other costs:
The cost to the employers of the state. Most states have laws on the books that on an election day they must allow voters to take some time off work to vote, thus hurting their productivity for several hours, potentially, per each employee.
The cost to employees. For people who work long hours and/or commute, having to schedule time to go to the polls (especially if they don't have a personal vehicle and commute by public transportation) is a Big Deal. Add in people who can't easily take those "legally allowed" extra hours off to vote and thus have to waste the time to vote out of their rare free time.
It's important to remember that ALL elections -- even those for Federal offices -- are a STATE matter.
When you vote for a candidate for a Federal office, you are not actually casting a vote for that candidate; rather, you are directing your state's legislative body to act a certain way on your behalf.
For Senators and Representatives, you are directing your state's legislature to appoint that person to the position for which they are running. This is why your state governor can fill vacancies in the House or Senate; because technically they sent the previous person there in the first place.
For the President, you are directing an elector from your state to cast a ballot in a national election, but you yourself are not voting in that national election. How those electors are chosen, and how they must vote is a matter of state law, not federal law.
In no circumstances are you ever casting a ballot in a national election. You are only casting a ballot for the state you live in.
Under this model, since federal elections are inherently state business, it only makes sense to also put other state business on the same ballot.