The reason they are forbidden by Allah Swt and Prophet Muhammad saw because they are harmful for the mankind. Allah Swt has said in the Quran that if a person avoids the Majors sins Gunah kabeera we will also forgive his minor sins due to it. Allah Swt says in Surah Al Nisa of Quran Kareem "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 many Major sins but i will mentioning some of the Major sins below.
1 Shirk - Associating partners with Allah.
2 Killing an Innocent Person.
3 Not offering Prayer.
4 Not Giving Zakat - (Charity to Poors)
5 Not keep Fast in Ramadan
6 Not doing Hajj
7 Doing Disrespect of Parents.
8 Doing Zina
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 Major sins Gunah kabeera but the above Major sins are very common in the society at present. May Allah Swt help us in avoiding Sins and doing the works of Virtue.