It's simple. The more cash you earn, the more Investors you'll attract! Your first Angel Investor will show up after you've earned 100 Trillion dollars on Earth. 
 
Earning Angels slower than you'd like? Try applying more bonuses, boosts, upgrades (Angel upgrades, too!) and multipliers. Remember, boosting your profits will also boost your Angels.
 
For the math geeks out there, the formula for earning Angel Investors is [150 * the square root of (Lifetime Career Earnings in quadrillions)] – Angel Investors spent. This means that when you earn an Angel Investor, it takes more money for you to earn your next one.
 
Are you a visual learner? Here's a fun video we created about earning Angel Investors: https://www.youtube.com/watch?v=EtsGp6b9OVI&t=87s