From the 1970s on, many cults are associated with religiously-motivated mass suicide. Why did this develop during the 70s--or was it in fact continuing an older tradition?
Your question is really two different, but related, questions. The first one is "how did cults become associated with mass suicide?" The second one is "did religiously-motivated mass suicides occur prior to the 1970s?" I can definitely answer the first, but I don't know the full answer to the second, though I'll try to point you to articles I've read that try to touch on it.
Note: in your original question, you referred to them as "cults." "Cults" isn't necessarily wrong, but it also has negative connotations in common context. This isn't necessarily ideal in the academic context, since having an inherent negative bias towards these groups would hamper proper examination of these groups. As such, this leads to debate as to what these groups ought to be called, whether any proposed new names are adequate enough, or whether they should be renamed at all. Alternatives to the word "cult" have been proposed in the literature, including "new religious group", "sect", "charismatic movement", and "marginalized movement." If you want to read more about this, I suggest checking out "'Cults': What They Are" in The Rhetoric of Religious Cults: Terms of Use and Abuse (Mooney, 2005).
Due to this debate, you might see these terms used in quotes from the literature. I'll personally be referring to them as marginalized religious groups in my answer, but note that if you see "NRM" or "sect" or "cult" in a quote or in an article/book I cite, it refers to the same thing.
How did marginalized religious groups become associated with mass suicide?
Basically, Jonestown happened.
The anti-cult movements (which I'll abbreviate as ACMs) of the 60s and 70s often warned about marginalized religious groups. These warnings generally claim that said marginalized religious groups are:
You can see many of these claims leveled against groups such as the Children of God, the Unification Church, Hare Krishna, the Church of Scientology, Jehovah's Witnesses, and (of course) Peoples Temple (henceforth PT).
A summary of PT is needed to understand how Jonestown became a darling -- if you could call it that -- of ACMs. As the group existed for over 20 years, this summary will be a bit long, so I apologize. A breakdown can be found in Raven: The Untold Story of the Rev. Jim Jones and His People (Reiterman, 1982), Understanding Jonestown and Peoples Temple (Moore, 2009), A Thousand Lives (Scheeres, 2011), and the well received documentary Jonestown: The Life and Death of Peoples Temple (2008). First-hand accounts also exist; see Stories from Jonestown (Fondakowski, 2013) and this list (presented without comment) for examples.
PT was a marginalized religious group, led by Jim Jones, that operated between 1956 to 1978. The group originated in Indianapolis as an interracial Christian church, and ran social service programs such as church-run nursing homes and a soup kitchen. They later became affiliated with the Disciples of Christ.
In 1965, after Jones warned of a nuclear holocaust that would devastate the midwest, the church moved to California, settling in Ukiah. They continued to do social service work in Ukiah and in San Francisco. Jones later expands the church, recruiting black people (often from other churches; many were poor, elderly, and/or otherwise felt dissatisfied with existing black religious groups such as the Black Church, Black Muslims, and the Nation of Islam) and white people (who were often middle-class and college educated), in San Francisco and Los Angeles. Jones preached anti-racism, advocated for "apostolic socialism" in his sermons, criticized the King James Version of the Bible for enabling racism and imperialism, and used healing sessions to great effect, causing them to gain members. PT also encourages members to live communally in PT-owned apartments, asking them to give up what they had with the promise of a stable allowance, housing, and food. The group also suffers from higher-ranking members choosing to leave the group around this time, most famously the "Eight Revolutionaries"/"Gang of Eight" in 1973, and Elmer and Deanna Mertle (later Al and Jeannie Mills) in 1975. Stories, accusations, and rumors of abuse, financial exploitation, and dodgy theology begin to circle the group, although for the most part they don't gain much traction in the media (outside an eight part series published in 1972, which ended prematurely after only four parts saw print). In 1973, PT begins work on an agricultural mission located in Guyana, in what is later known as Jonestown.
In 1975, after the election of George Moscone, Jones and PT begin to gain political influence. PT participates in political activism during this time, participating in letter writing campaigns and supporting the "Fresno Four" and the fight for the International Hotel (among others). In 1976, Jones becomes appointed to the San Francisco Housing Authority. Soon, Marshall Kilduff and Phil Tracy take interest in the group and Jim Jones in particular, and begin interviewing defectors, including new apostate Grace Stoen, who was now in a legal battle with her estranged husband, Tim Stoen, and PT, seeking custody for her son. Kilduff and Tracy publish their article in New West's August 1977 edition, leading to a mass exodus of PT members to Jonestown.
A dedicated ACM group, known as the "Concerned Relatives," begin to mobilize against PT. Concerned Relatives consisted of PT apostates (the "Gang of Eight", the Mills, Grace Stoen, Tim Stoen -- Grace's estranged husband, who now teams up with her in their custody battle -- in 1977, and Deborah Layton Blakey in 1978) and relatives of PT members. They lobbied government agencies, congresspeople, and the media to do something about Jonestown, who they claimed were holding their loved ones against their will in a "concentration camp." (See: Concerned Relatives flyer "This Nightmare is Taking Place Right Now", "Accusation of Human Rights Violations by Rev James Warren Jones Against Our Children and Relatives at the Peoples Temple Jungle Encampment in Guyana, South America", and the Affidavit of Deborah Layton Blakey).
After the exodus, tension begins to mount in Jonestown. The custody battle escalates in September 1977 when the Stoens' lawyer gets a court order from the Guyanese government requesting that Jones appear with their son, leading to an event known as the "Six Day Siege," where PT members armed themselves with machetes and guns and patrolled the grounds for six days. The battle stalls after PT has a talk with the Guyanese government, but the damage has been done. Jones's paranoia increases, and with a US Postal Service memo asking workers to return Social Security checks going to Guyana, a US Customs INTERPOL report revealing that Customs investigated shipments to Jonestown, and FCC investigations regarding amateur license violations for their HAM radios, members begin to believe that the US government, in tandem with Concerned Relatives, is set out to destroy their group as part of the Cold War fight against communism. They hire Mark Lane (a conspiracy theorist who believed such a conspiracy existed against PT) for legal counsel, begin practice suicide drills, and increasingly discuss suicide or violence against PT critics as part of educational exercises.
Ultimately, the Concerned Relatives got the attention of Congressman Leo Ryan, who decides to go on an expedition to investigate these claims. PT tries to prevent Ryan from going to Jonestown, but they ultimately relent on 17 November 1978, and allow him, his congressional group, members of the media, and members of Concerned Relatives to enter Jonestown.
Tragedy ensues on 18 November 1978. Sixteen people decide to defect from the group on that day, which upsets Jones. While the congressional group, the media, PT critics, and the defectors begin to board planes back to Georgetown, a PT group arrives and begins to shoot, ultimately killing five. PT members later gather in a central pavilion, where, after a prolonged discussion in which people express their fears of fascists coming in to kill their seniors and children and taking them back by force to the US, the group decides to self-destruct, primarily via poisoned Flavor-aid. One woman tries to argue against what was to happen during that discussion, but she fails to convince the group, and the self-destruction goes ahead. 909 PT members died that day, thus ending PT as a marginalized religious group.