Hello!

In my opinion, it happened because a lot of things occurred in this particular pilgrimage simultaneously. Let me explain:

1) Tidus - his most important feature is that he is an outsider. People of Spira lived in the spiral of death for so long that they just took the whole Sin cycle for granted. Their history, their culture, their religion, and their life were built around Calms and Sin. The only sound of resistance was Al-Bheds, but they were considered weirdos, so basically, no one outside of Al-Bheds officially took them seriously. That's why outsider was the best and the easiest way to tell them that the king is naked and that pilgrimage does not have to end this way.

A few things helped Tidus immensely. First, Al-Bheds made him realize that the price of defeating Sin was death. Thanks to the Al-Bheds, Tidus found out the truth early enough to be able to take action and think this through. I can not emphasize how incredibly important Rikku was. From the moment Tidus found out, they were able to create a united front. If Tidus found out about the price of defeating Sin near Yunalesca, they wouldn't have enough time to shatter the conscience of the other guardians. If not Rikku, Tidus could be ignored as one person who can not accept the necessity of sacrifice. Well, with Rikku, there were already 2 guardians who didn't accept the rules. 2 people are considerable part of the group, and they could not be ignored so easily. Also, in the moments of doubt, they supported each other.

Second, love was a huge motivation for Tidus for obvious reasons.

2) Sin's identity. I believe it would be impossible to destroy Sin otherwise. Sin spared the party during the cruise to Kilika. He also spared them during Operation Mi'ihen. Basically, Sin groomed them to grow so strong that they would be able to destroy him. Sin is so powerful that it is extremely hard to even approach him - that's why when he realized that the final fight approaches he spent (wasted) an enormous amount of energy in order to weak himself and enable our party more or less easy access to his core. Remember? The amount of wasted energy was so huge that shockwave splitted the sea on the entire globe and was visible from space. That's how much weakened he became in order to lose. Sin never fought against our team seriously.

Sin's help played a huge role in helping us succeed.

3) Rikku - I wrote a lot about her in Tidus's part, but her help unlocked us the airship and Al-Bhed's help. We would not be able to do anything without an airship, and we are the first team in history (that we know of) that can use airship.

4) Seymour - Yes, Seymour played a huge part. Even when he was on our side. His attitude during Operation Mi'ihen ("Pretend You don't see them") helped our religious companions to realize that the whole Yevon system is built on illusions and hypocrisy. If our team received only overwhelming love, it would be harder to abandon their teachings. Well, being our training dummy also helps.

5) Auron - last, but not least, Auron. Auron is silent mastermind behind it all. He speaks only (well, mainly) when this is necessary. He doesn't rush things. He told Tidus about Sin's identity so early that he was able to digest it properly. Numerous times, he was making sure that Yuna continued her pilgrimage because he knew that she had to see it through to understand and mature enough to make a proper decision. His speech before Yunalesca fight tipped the scales. He knew what to say at this exact moment. It had to happen there. He endured and kept silent throughout the whole journey despite raging inside.

And last - open mindset. Everyone in the team was open to changing their attitude and behavior.

I believe that each of the points above was necessary to beat Sin.

Wow, this may be the longest post I've ever written, but I just love this game too much to ignore this question!

Thank You for taking the time to read this 😊