Naku, medyo mahirap if we simply look at it based on statistics or just the numbers game - ang main factor kasi when it comes to having dirty/violent elections would be "corruption".
Think about it. Elections were made as a democratic means to elect leaders - someone speaks in a crowd; someone tells you about what he plans; someone engages in a debate. Ideally it should all be just those things.
Now put "corruption" in the equation - whether it's a Roman tribune paying off mobs to turn violent, or a Filipino politician engaging in vote-buying - kumbaga these are being done since time immemorial - then things get messy.
So what exactly was the major difference in pre and post ML times?
Corruption, and by extension, dirty politics and elections - did exist before ML. Hell, it was even widely circulated that Magsaysay (aka. "the guy whom gradeschoolers were taught as the shining beacon of PH politics when he was president) - had a CIA handler who can boss him around (in relation to what u/gradenko_2000 said). Vote buying/bribes did exist, as did the use of violence as well... kasi nga, these tactics have been used since 'time immemorial.
BUT... many of these were done covertly and/or sporadically. In some rural areas, you could run for public office by simply saying you were - a good neighbor - nandun pa rin ang idealism that elections can be fair, clean, and for the greater good.
Then Marcos/ML came around - we had the actual elections before that, then the snap elections/ousting at EDSA much later, as well as every event and incident in between.
The idea that corruption existed but was practiced covertly and sporadically was... gone. POOF!
Vote buying, yep; violence, yep; manipulating results, yep; bribes to gov't officials, yep; hoarding wealth, yep; cronies, uhuh... and many more.
While Marcos was not the first to do these things - his administration was the first to do it so openly and brazenly... for a prolonged period of time that it was in power.
And then, plot twist, the Marcoses were back in power in local/national politics just a few years later.
So the average Filipino's line of thinking went from: "some politicians may be corrupt" -to- "all politicians are corrupt.
Here's a quote from Richard Nixon after Watergate (if you watched the movie Frost/Nixon, you'll know what this is):
I let down my friends, I let down the country, I let down our system of government and the dreams of all those young people that ought to get into government but will think it is all too corrupt and the rest.
When you do something corrupt so openly and so brazenly, out in full view of everyone - it sends out a signal to a lot of political clans and even aspiring politicians that: "Hey, I know it's wrong, but some people can get away with it for a long time, maybe I can as well..."
In many ways, that idealism of fair and clean politics is replaced by the harsh reality that politics is corrupt and dirty - and that becomes an accepted norm.
Do a lot of people dislike that it's like that - obviously. But for the most part, many of us just think - "Ah, well, ganyan talaga..." with apathy.
Sorry napahaba ang sagot ko but yeah, essentially, it's the mentality and open characterization about politics and elections that drastically changed after ML/Marcos.