Your Angel Investor total directly correlates to your all time cash earned. Earning Angels slower than you'd like? Try applying more bonuses, boosts, Upgrades (Angel Upgrades too!) and multipliers; boosting your profits will also boost your investors.
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.