Too many crooks spoil the rip-off?
Maybe the pump-and-dumpers are finding that bigger fish are swimming in and consuming their profits.
There are hundreds of investment houses around the world, earning their bread and Colombian frosting by trying to work out which shares are going to rise or fall. In between liquid lunches and frequent nostril-to-desktop action, they scan the markets and news across the world, looking for anything that might affect share prices. For such an establishment, it would be worth setting up a standalone PC with a few spam-magnet email accounts, and getting someone junior to collate together the pump-and-dump emails and provide hourly reports to the traders, with trends.
Professional traders in an investment house would probably have better access to the markets, and could probably beat the spammers to the punch when it came to selling off the inflated shares. Or, as the shares picked for pump-and-dumps are usually inferior performers and unlikely to recover quickly after the pump-and-dump treatment, the investment house might just decide to sell off any holdings they have in that company immediately. It wouldn't take many such investment houses to make the value of a pumped share collapse before any ill-gotten gains could be reaped from it.
Ain't life just tough all over!