Since they got a guilty plea out of him, I'm guessing they had strong enough proof to discourage him from going to trial where he could've risked up to 25 years

Since they got a guilty plea out of him, I'm guessing they had strong enough proof to discourage him from going to trial where he could've risked up to 25 years in jail time if jury found the evidence credible. If he had just stuck with stocks, I'm sure it would've been harder to prove. But with options and dates, timing of it... it's hard to get away.