Becuase its basically by the rulebook cheating.
Always was. It was just counted on to be too difficult to make a difference. Now it is, and its killing the rest of the game. Don't believe me? Watch these numbers:
Since 1980, year by year, average number of 2pt shots a game, 3pt shots a game, and % of all shots that were threes:
1979-80: 87.88 2PA, 2.77 3PA = 3.1%
1980-81: 86.40 2PA, 2.02 3PA = 2.3%
1981-82: 85.96 2PA, 2.28 3PA = 2.6%
1982-83: 87.40 2PA, 2.26 3PA = 2.5%
1983-84: 85.98 2PA, 2.38 3PA = 2.6%
1984-85: 85.96 2PA, 3.13 3PA = 3.5%
1985-86: 85.29 2PA, 3.34 3PA = 3.8%
1986-87: 84.06 2PA, 4.73 3PA = 5.3%
1987-88: 82.71 2PA, 5.00 3PA = 5.7%
1988-89: 82.41 2PA, 6.55 3PA = 7.4%
1989-90: 80.55 2PA, 6.60 3PA = 7.6%
1990-91: 80.05 2PA, 7.15 3PA = 8.2%
1991-92: 79.72 2PA, 7.63 3PA = 8.7%
1992-93: 77.00 2PA, 8.95 3PA = 10.4%
1993-94: 74.55 2PA, 9.89 3PA = 11.7%
1994-95: 66.18 2PA, 15.30 3PA = 18.8% Shortened Line Experiment (22 foot arc)
1995-96: 64.13 2PA, 16.05 3PA = 20.0% Shortened Line Experiment (22 foot arc)
1996-97: 62.50 2PA, 16.79 3PA = 21.2% Shortened Line Experiment (22 foot arc)
1997-98: 67.00 2PA, 12.70 3PA = 15.9%
1998-99: 65.04 2PA, 13.16 3PA = 16.8%
1999-00: 68.38 2PA, 13.72 3PA = 16.7%
2000-01: 66.89 2PA, 13.71 3PA = 17.0%
2001-02: 66.52 2PA, 14.74 3PA = 18.1%
2002-03: 66.10 2PA, 14.68 3PA = 18.2%
2003-04: 64.89 2PA, 14.93 3PA = 18.7%
2004-05: 64.59 2PA, 15.76 3PA = 19.6%
2005-06: 63.01 2PA, 15.98 3PA = 20.2%
2006-07: 62.77 2PA, 16.94 3PA = 21.3%
2007-08: 63.39 2PA, 18.11 3PA = 22.2%
2008-09: 62.79 2PA, 18.12 3PA = 22.4%
2009-10: 63.57 2PA, 18.13 3PA = 22.2%
2010-11: 63.21 2PA, 18.01 3PA = 22.2%
2011-12: 63.05 2PA, 18.38 3PA = 22.6%
2012-13: 62.00 2PA, 19.95 3PA = 24.4% <-------------- Explosion begins
2013-14: 61.46 2PA, 21.54 3PA = 25.9%
2014-15: 61.15 2PA, 22.41 3PA = 26.8%
2015-16: 60.49 2PA, 24.09 3PA = 28.5%
So when do you sound the alarm? When it hits 1/3 of all shots taken? At the current pace of expansion, that should be 3 years. 1/3 of all shots will be threes. Or do you wait until 40%? 50%? Or do you recognize you have a perverse incentive structure causing an issue and work to head it off before it entirely supplants the rest of the sport?
I refer you to the mid-90s experiment with a shortened 3pt line. It was too effective, causing too big a perversion of the game. Took a decade for threes to catch back up to hose levels after the experiment ended, and until 5 years ago they were holding pretty steady in the low 20% range. High, but seemingly stabilized. But in the last 4 years this explosion has been unprecendented, and it is rapidly shoving out every other approach to the game. The mid-90s explosion showed one thing clearly: this is all about incentives. If you de-incentivize 3pt shooting, you can cut back on its prevelance, carve some room back out for the drivers, the post players, the mid range shooters. But if you don't, the enormous +50% bump you give to 3pt shooting makes it the ultimate weapon, and its rapidly turning a deep and complex sport into a one trick puppy.