I suspect writing F&B took a lot of his time, as in, he probably put TWoW on hold while he was working on it. I also suspect he has decided to change course once the show outpaced him, since he thought it wouldn't happen, so the plot has probably changed in ways that required rethinking or rewriting a lot.
This is also somewhat likely. The show's reception (good or bad) likely caused him to alter or scrap plotlines he might have been considering, lengthening the delay. TOO close to the show and hardcore fans will be pissed. not close enough to the show and casual fans will be bewildered.
But just for fun- here is a ASOIAF timeline, assuming you picked up the first book in 1996.
1996- "A GAME OF THRONES" IS RELEASED, DELIGHTING FANTASY FANS EVERYWHERE
1996- William Jefferson Clinton is re-elected as president. Al Gore is re-elected Vice President.
1996- The Nintendo 64 is released in the United States.
1996- Atlanta Hosts the Olympic Games
1997- Princess Diana is killed in a car crash
1997- OJ Simpson found "liable" for the death of his wife in a civil trial.
1997- J.K. Rowling Publishes the first Harry Potter book
1998- "A CLASH OF KINGS" IS RELEASED, DELIGHTING FANTASY FANS EVERYWHERE
1998-Bill Clinton is accused of having sex with Monica Lewinsky
1998- Google introduces their search engine to the web
1998- Viagra invented
1999- Two teenagers kill 15 students including themselves at Columbine High School
1999- World Population hits 6 billion
1999- Dow Jones closes above 10,000 for the first time ever.
1999- Author Steven Erickson releases "Gardens of the Moon" the first book in the Malazan book of the Fallen.
2000- "A STORM OF SWORDS" IS RELEASED, DELIGHTING FANTASY FANS EVERYWHERE
2000-Y2K scare turns out to be nothing
2000-George W. Bush defeats Al Gore for presidency of the united states.
2000- The Dot Com Bubble bursts
2000- The Playstation 2 is released in the United States
2000- Athens hosts the olympic games
2001- Terrorists Attack the World Trade Center in New York, destroying it
2001- Enron corporation files for bankruptcy
2001- 50% of Americans now use the internet.
2001- Apple's "ipod" becomes the best selling MP3 player in the world
2002-American Idol Debuts on Fox
2002- The Wire Debuts on HBO
2002- The Euro is invented and enters circulation as currency
2002- Sam Raimi's "Spider Man" becomes a box office hit
2003- George W. Bush Attacks Iraq.
2003- Saddam Hussein Captured in Iraq.
2003- Arnold Schwarzenegger Elected Governor of California
2003- Space Shuttle Columbia explodes.
2004- Enormous Tsunami hits asia, killing 225,000
2004- Facebook Invented
2004- George W. Bush Re-Elected
2005- "A FEAST FOR CROWS IS RELEASED, DELIGHTING FANTASY FANS EVERYWHERE"
2005- Hurricane Katrina devastates the US gulf coast, causing 81 billion in damage
2005- Youtube posts it's first video
2005- Author Brandon Sanderson releases "Elantris" his first novel.
2005- Pope John Paul Dies, Succeeded by Pope Benedict XVI.
2006- Playstation 3 is released
2006- Steve Irwin, Crocodile Hunter dies in a freak accident
2006- Twitter Invented
2006- Saddam Hussein Executed
2007- Subprime mortgage collapse begins, wiping out billions and launching the US into a recession
2007- Nancy Pelosi becomes the first female speaker of the house of representatives
2007- Apple Computer releases the iPhone.
2007- J.K. Rowling releases her seventh and final Harry Potter novel
2008- Barack Obama makes history, as he is elected the first black US president
2008- Beijing hosts the Olympic Games
2008- "Iron Man" released by Marvel Studios, becoming the first film in the MCU and grossing a whopping $585 million dollars worldwide.
2009- Tea Party is invented, protests break out across the United States
2009- Michael Jackson Dies
2009- Global Financial Crisis is official
2010- Obamacare created by US government
2010- 200,000 people killed in Haiti Earthquake
2010- Deepwater Horizon explodes in the Gulf, spilling 4.9 million barrels in the worst spill in industry history.
2010- Wikileaks begins releasing classified documents
2010- "Don't Ask, Don't Tell" officially ends as US policy
2011-" A DANCE WITH DRAGONS" IS RELEASED, DELIGHTING FANTASY FANS EVERYWHERE
2011- Osama Bin Laden killed by US forces
2011- "Game of Thrones" debuts it's first season on HBO.
2011- Steve Jobs Dies
2011- World Population Hits 7 Billion
2011- Occupy Wall Street Movement becomes a thing
2011- Author Steven Erickson releases "The Crippled God" the tenth and final book in the Malazan Book of the Fallen.
2012- Barack Obama re-elected US president, defeating Mitt Romney. Joe Biden just happy to be there.
2012- London hosts the Olympic Games
2012- Sandy Hook Elementary School shooting occurs
2013- the Playstation 4 is released
2013- Boston Marathon Bombing Occurs
2013- Nelson Mandela Passes Away
2013- Pope Benedict Resigns, succeeded by Pope Francis
2013- Edward Snowden leaks classified documents, flees US.
2014- 1 World Trade Center opens in NYC
2014- US restores full diplomatic relations with Cuba since 1961
2014- Ebola Epidemic becomes global health crisis
2014- Comedian Robin Williams dies
2015- Obergefell vs. Hodges Legalizes Same Sex Marriage in all 50 US states
2015- Former Subway spokesman Jared Fogle sentenced to prison
2015- China scraps the "one child" policy.
2016- Britain Votes to leave the European Union
2016- Donald Trump defeats Hillary Clinton to become US President
2016- Rio Hosts the Olympic Games
2017- Stephen Paddock opens fire in Las Vegas, injuring 851 and killing 53.
2017- Author Brandon Sanderson releases "Oathbringer" his 27th novel.
2017- The Philadelphia Eagles defeat the New England Patriots to Win Super Bowl LII
2017- Black Panther, Infinity War, and Ant Man and The Wasp release, becoming the 18th, 19th, and 20th films in the MCU and grossing nearly 4 billion dollars in a single year.
2018- BOOK 6, "
THE WINDS OF WINTER" STILL NOT RELEASED, TO THE FRUSTRATION OF FANTASY FANS EVERYWHERE