Hinduism....Why is it repeatedly mentioned in our Puranas that...?

'Hearing this story of so and so, one shall be cleansed of all his sins and will attain to salvation'. Why such lines are added. I have read those stories with great attention and devotion so many times. And if it is true then i should have got salvation maybe at-least 100 times. What about you? Any one attained salvation?

Hinduism say "DHARYETI ITI DHARMAH". What you do earnestly is your religion. The main thing is having FAITH. You need not read or hear different stories . Had you followed the principles of at least one of them you'd have felt that you are more purified than others.
These things are added to create more spiritual;ism and more respect to the religion. They say if you take a dip in the Ganges all your sins are washed away. There also Faith play the important role.
Let me tell you that there are Crores of Deities and Gods and Goddesses in Hinduism. The main thing is to select one out of them and pin your entire belief in him/her. Do not falter at what others say and you'll see instant and prompt results of your belief. That's the key > Faith.

It is called phyla Surat, the promise of success given to one who hears it.

By regularly hearing, chanting and meditating on the beautiful topics of Lord Mukunda with ever-increasing sincerity, a mortal being will attain the divine kingdom of the Lord, where the inviolable power of death holds no sway. For this purpose, many persons, including great kings, abandoned their mundane homes and took to the forest.

PURPORT

For the Srimad-Bhagavatam's Tenth Canto, this verse is the phyla-sruti, the promise of success given to one who hears it. The process of devotional service begins with hearing topics about the Supreme Lord. When one has heard these topics properly, he can then proceed to chant them for others' benefit and reflect on their significance. This leads to faithful adherence to the principles of devotional service, which culminates in absolute faith in Lord Krsna. Such perfect faith gives one the right to enter the Lord's intimate service and, in due course of time, return to ones eternal, spiritual life in one of the Lord's personal domains.

Humbly offering his comments on the Tenth Canto at the lotus feet of his worship able Lord, Srila Visvanatha Cakravarti prays,

mad-gavir api gopalah

svi-kuryat krpaya yadi

tadaivasam payah pitva

hrsyeyus tat-priya janah

"If Lord Gopala mercifully accepts the cows of my words, then His dear devotees may enjoy the pleasure of drinking their milk -- the nectar produced by hearing them."

http://vedabase.net/sb/10/90/50/en2

Forget about Purans, learn and practice philosophical Hinduism (Vedanta), start with Swami Vivekananda.

During those times,that was a way to motivate people to listen to the endless stories in the Puranas.