Well, it is all pretty much explained in the movie. Also, a few points should be noted:
This all happened between 1989 and 1996:
Stratton Oakmont operated between 1989 and 1996, this was the pre-WWW era as we now it (no social networks, no review websites that will tell you what's good and what's bad, ...), so the word-of-mouth won't have as much impact as it would nowadays.
And most importantly,
Stratton Oakmont used "hard sell" techniques:
They recruit several of Jordan's friends, whom Jordan trains in the art of the "hard sell". The basic method of the firm is a pump and dump scam. To cloak this, Jordan gives the firm the respectable-sounding name of Stratton Oakmont.
What is a "hard sell"? From Wikitionary:
A sales technique of pressuring the potential buyer to agree to a purchase.
What is "pump and dump"? From Wikipedia:
"Pump and dump" (P&D) is a form of securities fraud that involves artificially inflating the price of an owned stock through false and misleading positive statements, in order to sell the cheaply purchased stock at a higher price. Once the operators of the scheme "dump" sell their overvalued shares, the price falls and investors lose their money. This is most common with very small corporations, ie "microcaps."
While fraudsters in the past relied on cold calls, the Internet now offers a cheaper and easier way of reaching large numbers of potential investors through spam email, bad data, social media, and fake news.
What are "cold calls"? From Wikipedia:
Cold calling is defined as the solicitation of business from potential customers who have had no prior contact with the salesperson conducting the call. Cold calling is used to attempt to convince potential customers to purchase either the salesperson’s product or service. Cold calling is generally referred to as an over-the-phone process, making it a source of telemarketing, but can also be done in-person by door-to-door salespeople. Though cold calling can be used as a legitimate business tool, scammers can use cold calling as well.
The last paragraph says it all. This is exactly what we've seen happen throughout the entire movie: Stock brokers at Stratton Oakmont cold-calling people and persistently persuade them to buy the stocks.
Here, Jordan Belfort explains it himself: