This is a great question.
It's old, but I'll answer it anyways because it's really important for all entrepreneurs, newbies, experienced and veterans to know.
There are really tons of examples of side projects which turned out to be highly successful companies.
In addition, there are thousands, if not more, of examples of entrepreneurs who just started doing something as a side project and ended up building an enterprise out of it.
In fact, this is one of the greatest ways a company can be built, because the market's demand is actually urging you to focus on your side project so much, that it eventually ends up being the main one.
Because you asked how did the founders decide it was time to focus on the side project and drop the main project, I'll answer with a few real-life examples.
Close.io started exactly as that, as a side project.
The company (Elastic, Silicon Valley based) had already run other "main" projects before and some of them had, in fact, found success.
The same team, before it did Close.io (Steli if you are reading this thanks for the inspiring insights), went through Y-Combinator with a totally different idea-project.
The funny thing is that they already had success. They were growing like crazy and they even raised funding after YC's demo day.
Now, after their growth curve raised, before the fundraising round, when they raised funds, their curve droped horizontal. Funny, right?
You just don't know when success (or failure) is going to hit you.
Now, here's the thing:
That project failed and the startup closed.
But, the same team, with the same core went on to build a different project.
Within that project and because the team wanted to find a way to manage its sales better, they built a software (for themselves), called Close.
Today, close.io is a growing, very successful SAAS startup.
So, what happened here?
After a series of ups and downs (we are talking about a 6-8 year process), failures and successes, the same team (! important ! ) built something for themselves but it turned out this piece of software they built brought actual value to the mass.
So, they ended up selling this software as SAAS and building a successful startup on it.
Another great example is Twitch. Today you may see the result, Twitch being sold for almost $1 billion (with a B) but
I guess you all remember Justin.tv (Justin Kan is another great inspiration for me).
Do you remember when justin.tv first launched?
And what it did offer?
It was almost 8 years ago and what it initially was about was Justin's 24h daily life, streamed live.
Not that fancy, huh? I guess you can't see the $$$ pouring out and the users lining up to see how a (then) nobody's 24h life was.
But the same team (I believe the core remained the same), went on to iterate, pivot multiple times. Again, it's a process of years.
Firstly they allowed everyone to stream, instead of just Justin.
Then they opened a bunch of categories.
Gaming soon became very popular and a few years later, here's Twitch.
What happened here, again, is that the customers or users , a.k.a. the marketplace, defined what's successful.
When Justin started, I doubt he ever thought that Justin.tv would end up being a game-streaming platform. Gaming was just one of the "side-project-categories" which was added at a later time. And it ended up being a huge success.
Short words: When it's time for you as a founder to abandon you "main project" you will know. You will understand.
If you are smart enought to find you way here and post a question you will most probably be able to understand when the time is right.
By the way, there is no such thing as a main project for a startup, as you should alwasy be searching for what the market wants.
I hope that helps.
If I can help any other way, you can ask me through a message or a call.
PS: Y-Combinator always asks its applicants to post a side-project idea, because it may end up being the main one.
Does this ring a bell?