In scooping up the U.S. retail banking operations of Royal Bank of Canada for $3.45 billion in June, Pittsburgh's PNC Financial Services Group Inc. is doing something its big-bank brethren have not engaged in over the past two years: big-ticket acquisitions.

The notable absence of blockbuster deals from Citigroup Inc., J.P. Morgan Chase & Co., Bank of America Corp. and Wells Fargo & Co. -- at a time when others have jumped back into the M&A fray -- has stirred talk about when they will return. "I'm trying to figure out what trips the trigger for these guys to do a deal," says a senior M&A banker.

Their absence has been all the more surprising, considering that the financial sector has topped other industries in M&A volume since the start of 2009, making up fully 25.9% of all deals done from 2009 through Aug. 29. Second in the rankings is healthcare, at 12.9%, about half the deals of finance, according to Dealogic.

Who's been busy? A glance at the list of the top dealmakers in the financial sector reveals an awful lot of private equity and investment firms such as BlackRock Inc., Blackstone Group LP, TPG Capital, Carlyle Group (which filed for an IPO after Labor Day weekend) and Oaktree Capital Management LP. Their activity has been fueled by easy access to capital and driven by poaching top dealmakers with corporate expertise. One recent example: In late August, Warburg Pincus hired Charles Carmel, Cisco Systems Inc.'s top dealmaker, as a managing director.

In contrast, the biggest banks have been hamstrung by concerns about pending financial and capital regulations, a weak economy and volatile share prices. Some big banks still seem to be digging out from problems associated with the megadeals of 2008, notably Bank of America, with its acquisitions of Countrywide Financial Corp. and Merrill Lynch & Co. Many are struggling with a pile of bad assets and litigation, mostly related to real estate.

"Typically after a recession, you begin to see premium bank M&A once inflows of nonperforming assets slow down," says David Hilder, senior analyst with Susquehanna Financial Group LLLP in New York. "We've seen this slowdown for the last six quarters, and this is when you would expect to see more deals."

Others suggest the banks may be experiencing a more fundamental shift in strategic differentiation. Some traditional super-regional banks, like PNC Financial, have been building critical mass. PNC's chief dealmaker, CEO James Rohr, has developed a reputation for pulling off savvy deals over the past decade. He has been credited with a knack for spotting undervalued or distressed assets that he grabbed below book value, such as the $5.2 billion acquisition of Cleveland-based regional rival National City Corp. in the midst of the meltdown.

PNC's RBC acquisition now makes it the fifth-largest bank by assets and deposits in the U.S. "PNC got there doing good deals," notes Chicago-based Morningstar Inc. analyst James Sinegal.

As for J.P. Morgan, BofA and Wells Fargo, he says, "I think their dealmaking is done for a while." That's at least in part because the three banks are bumping against the 10% deposit threshold, which would create legal problems if, as speculation suggested in late August, J.P. Morgan was to suddenly take over BofA.

Besides, the big banks have lots of dealmaking to live down. Critics argue they suffered from empire-building dreams that too often backfired. BofA bought Countrywide only to find it had inherited more toxic mortgages than it originally imagined. "The majority of BofA's problems have come from bad deals. Their potential to do more deals is essentially zero, which is probably a good thing," declares Sinegal.

Others do praise BofA for working to clean up the bank through divestitures, write-offs, cost cutting and capital raising. On Aug. 30, the bank said it plans to divest its 5.2% stake in China Construction Bank Corp. for $8.3 billion, the bank's second-biggest divestment on record. And, of course, it picked up a $5 billion shot of confidence from Warren Buffett's Berkshire Hathaway Inc.

For many of the big banks, divestitures have ruled. In 2010, divestitures for top banks such as J.P. Morgan, Citi and BofA surpassed acquisitions on average by roughly 2-to-1 in some cases, according to Dealogic.

But no trend lasts forever. Some bankers are watching closely for this one to turn. "We have some well-capitalized banks in a pretty good position to drive consolidation at some point," says Scott Humphrey, head of BMO Capital Markets Corp.'s U.S. mergers and acquisitions group. "The question is, what's going to be the kindling to start that blaze?"