Pacquiao, 37, has not fought since he was convincingly beaten on points by Floyd Mayweather in May.
Promoter Bob Arum says Pacquiao and WBO welterweight champion Bradley, 32, will meet in Las Vegas on 9 April.
Khan also hoped to fight Mayweather in 2015 but the American great retired after beating Andre Berto in September.
Khan, 29, only fought once in 2015, outpointing Chris Algieri in May, and looks set for a clash with Sheffield's IBF welterweight champion Kell Brook in the summer.
"He has a spotty record, hasn't done great ratings, Arum told the Los Angeles Times. "Why would we feel he'd do well on pay-per-view?"
Bradley was awarded a controversial decision against Pacquiao in 2012 before the six-weight world champion earned revenge in 2014.
Arum claims he has agreed deals with both fighters, though the paperwork for the bout has yet to be finalised.
The Bradley fight could be Pacquiao's last, although Arum refused to confirm that would be the case.

Twin B and Yasmin Evans announced on 1Xtra that the R&B singer, who's influenced the likes of Alicia Keys and Rihanna, will be playing at 1Xtra Live.
It's the seventh year the flagship gigs have taken place and this year it's all happening in Birmingham.
Mistajam and DJ Target will be hosting shows live from the National Indoor Arena (NIA) on BBC Radio 1 and 1Xtra on 15 November.
Mary J Blige has won multiple Grammy awards, was ranked ninth in music channel VH1's list of the 100 greatest women in music (Madonna was number one, if you're wondering) and has sold millions of records around the world.
Next month we'll get to hear some of her most famous songs live, including No More Drama, Family Affair, Real Love and I'm Going Down.
She joins a 1Xtra Live line-up that already includes Rick Ross, who'll be performing his first ever live show for the BBC.
Jess Glynne, Fuse ODG and Lethal Bizzle will also be on stage.
George Ergatoudis, Head of Music, Radio 1 and 1Xtra, said: "There are not many events in the world that can boast having both Mary J Blige and Rick Ross.
"For 1Xtra Live we've pulled together a bill featuring 1Xtra favourites from both the UK and abroad - Birmingham is in for an amazing night of live music."
Joining the stars at the NIA will be 1Xtra DJs like Trevor Nelson, Charlie Sloth and Sarah Jane Crawford.
There are 10,000 tickets available for the show.
Tickets will go on sale on a first come first serve basis at 18:00 BST on Thursday 23 October.
They'll cost Â£10 each plus handling fees and you have to be 16 or over to attend. Full terms and conditions are available on the 1Xtra website.
If you can't make it to the Midlands, the show will be available to watch on the Red Button and will be streamed on the 1Xtra website.
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Corbridge in Northumberland was hit by Storm Desmond in 2015, with more than 70 homes evacuated as the River Tyne reached record levels.
Water drainage near Station Road and Tinklers Bank was now being improved, Northumberland County Council officials said.
Cabinet member Ian Swithenbank said the project was "vital".
"We've been working tirelessly since the severe weather last winter to clean-up and repair roads and infrastructure both in Corbridge and the wider area and this major piece of work will help protect the town in the future," he said.
A new outfall through Tynedale Rugby Club will replace a culvert badly damaged during the storms, the authority said.
Club chief executive Neil Foster said flooding at the ground seemed "insignificant compared to the impact on local residents".
"Hopefully this new surface water drainage system will reduce the risk of future surface water flooding, particularly in Station Road where flooding has historically been a problem," he said.
The work is expected to take up to four months.
Britain's Amir Khan has missed out on a fight against Manny Pacquiao, with the Philippine great choosing a third match with American Timothy Bradley instead.
[NEXT_CONCEPT]
Sigma, Fuse ODG, Tinashe and now Mary J Blige.
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Work has started on new flood prevention measures for a repeatedly flooded village.