If you make it free you will always get more people to use your products. This helps a lot in getting funding in case of startups. Look at our user base if each user paid 1 dollar we would make billions. Of course you can never turn 100% of your user base into a paying customer.
If you want to keep your massive user base you have to resort to ads. If you switch to a paid plan your user numbers will decline and investors will complain. If you do not show ads you will continue to bleed money and make no return on investment.
At some point in time having a small paying customer base became unsexy to a lot of investors who wanted to chase after the next potential unicorn.