Short answer is: they didn't really, they just evolved with the times.

As well as the activities (like strikebreaking) that made them notorious, the Pinkerton National Detective Agency was also a favoured government contractor for investigative work at a time when the federal government lacked a national, general-purpose criminal investigative agency (the Bureau of Investigation, which became the FBI, was only created in 1908.)

The use of the Pinkertons as outsourced federal law enforcement only came to an end in 1893 when Congress passed legislation (see 5 USC 3108) prohibiting the use of detective agencies by the federal government:

§3108. Employment of detective agencies; restrictions

An individual employed by the Pinkerton Detective Agency, or similar organization, may not be employed by the Government of the United States or the government of the District of Columbia.

Their role changed again as the US labour movement matured — their labour spying activity was investigated by Congress in the late 1930s (the La Follette Committee), which caused a public relations problem for the agency. Over time, they shifted focus into protective services/corporate security.

Essentially, as America became less of a frontier nation, with a more mature civil society and a more sophisticated law enforcement system, the need for (and trust in) private detectives dwindled — meanwhile, the rise of corporate America left plenty of room for 'risk management' and security consultants. So that's what they became.

The Pinkerton brand still exists today. They were acquired by Securitas, one of the world's largest private security firms, in 1999, and now operate as a subsidiary company.

I haven't read it, but it looks like your best source for a more detailed history would be The Eye That Never Sleeps: A History of the Pinkerton National Detective Agency by Frank Morn. It was published in 1982, though, so doesn't follow the most recent phase of their evolution.