Explaining this is going to take a fair bit of background. To start with, the OP's comment "In practice, we always have the elections at the same time," isn't accurate. At the time of the proposed constitutional referendums for simultaneous elections (they failed twice,) the elections for the two houses of parliament had been out of step since 1961. Of forty four referendum proposals to amend the Constitution only eight have passed. It might be more interesting to ask why the 1977 attempt came closer to passing than the same proposal in 1974.

How are the elections for the House of Representatives and the Senate different, and why?

Apart from casual vacancies, all members of the House of Representatives are elected at the same time, from electorates that are based on population, and by preferential voting. I.e. Each electorate supplies one member of the House of Representatives who must have more than 50% of the vote in the electorate. They have a maximum term of three years although elections may be held more often and effectively the party in power can pick an election timing that benefits them. For this reason they do not always align with the Senate elections.

Apart from casual vacancies, members of the Senate have a six year fixed term but half-Senate elections are held every three years so their terms overlap,. (the four Territory members only have three year terms.) Each State is an electorate and provides twelve Senators per state. The Senators are elected by proportional voting of all voters in the State.. This means that each Senator, notionally, represents all the members of his/her state. The Senate is intended to act as a check upon the House of Representatives and voters often reflect this in their voting patterns, strategically voting differently in Senate elections than they do in House elections. This, combined with the proportional vote and the issue of only half of the Senate being up for election every three years means that the Senate is more likely to contain members from minor parties and have a different party balance than the House. Since each State provides the same number of Senators despite population levels, the less populous States view the Senate as their protection against the competing interests of the more populous States which have proportionally more members in the House.

In Australia, the Commonwealth Government has whatever powers are granted to it explicitly or implicitly by the Australian Constitution. All other powers remain with the States. This means that any change to the Constitution is likely to change the power balance between the States and the Commonwealth. The Constitution can only be changed by a referendum approved by an absolute majority of voters and an absolute majority of States but the only effective way to propose a referendum is for the Commonwealth government party in power to propose the referendum. This means that any change in power between the Commonwealth and the States resulting from the proposed change is likely to be in the Commonwealth’s favour. The actual proposal is called the ‘Yes Case’. Any parties opposing the proposal provide a ‘No Case’. Details of both cases must be published by the Australian Electoral Commission before the referendum. The only referendum for which no-one prepared or published a ‘No Case’ was the 1967 proposal to remove the restrictions on the Commonwealth from making laws effecting aborigines and counting them in the Commonwealth census, and which was approved overwhelmingly.

So, getting back to the original question although slightly restating it; Why did the same proposal for Simultaneous Elections come closer to passing in 1977 than it did in 1974? What was the ‘Yes Case’ and what was the ‘No Case’ and why did the State Governments of the less populous States, Queensland, Western Australia and Tasmania, campaign against it.

Warren Keir, (Referenced below) provides summaries of Yes and No cases for all constitutional referendums and a detailed analysis of voting patterns. He notes a significant effect based on the political party identification of the voters although he notes other, competing effects for instance when one party held Commonwealth government but an opposing party held State governments.. Of the two referendums run in 1974 (one of them being Simultaneous Elections), and proposed by the Whitlam Labor Government which then held power, but opposed by the Liberal alliance, most supporting votes came from safe Labor seats however both failed. Of the four referendum proposals held in 1977 by the Fraser Liberal Government and supported by the Australian Labor Party, three passed and the Simultaneous Elections proposal which was a duplicate of the earlier proposal came much closer to passing but failed.

The summary of the ‘Yes Case’ was that if the elections remained out of alignment it was possible to have four Federal elections in four years and requiring simultaneous elections was intended to reduce the cost and time involved in having frequent elections.

The summary of the ‘No Case’ was that this was an attempt by the Commonwealth Government to politically align the membership of the Senate with the House by encouraging voters to vote the same way at the same time, as by forcing the vote for the Senate to occur simultaneously with the vote for the House, where the party in control of the house could select the timing of the election to their benefit and that this would reduce the protection the Senate gave the States.

Since the less populous States, which had lower representation in the House than in the Senate saw a greater need for the protection of the Senate it was those State Governments which most strongly opposed the proposal. The greatest opposition to the proposal was in Tasmania, which had the smallest population.

References:

Parliament of Australia, various publications for description of voting systems

Warren Keir: Voter Behavior and Constitutional Change In Australia Since 1967, Phd Thesis, Queensland University of Technology.