The real reason these sins are forbidden by Allah Swt and Prophet Muhammad saw because they are harmful to the mankind. Allah SWT is our creator and he knows best what is good and what is bad for us. If a person avoids the Majors sins (Gunah e Kabira) his minor sins will also be forgiven by the mercy of Our Rab. Allah Swt says in Surah Al Nisa of Quran.
"If you avoid the major of what you have been forbidden to do, We will cancel out for you your other evil deeds and will admit you to Heaven with a noble entry"
There are several Major sins but I will be mentioning some of them below not all of them.
1 Shirk - Associating partners with Allah SWT.
2 Killing an Innocent human being.
3 Not offering Fard Salah Prayer.
4 Not Giving Zakat - (Charity to Poors)
5 Not keep Fasts in Ramadan
6 Not doing Hajj despite having money
7 being Disrespect of Parents.
8 Doing Zina
9 taking or giving Interest
10 Lying about Allah Swt and his Prophets
15 Doing Suicide
16 Earning Wealth by unlawful means.
17 unjust Judging
18 Bad conduct towards your life partner
19 Plotting the Evil.
20 treating someone unjustly
21 False Oath taking.
There are many other big sins as well but the above ones are very common in the society at present. May Allah Swt help us in avoiding these Sins and doing the works of virtue in our lives.