Exactly the same reason that the Thatcher govt. undermined unions, strike action, and especially (re)banned sympathy strikes/solidarity action: They fucking work.
They have taken untold millions, that they never toiled to earn;
But without our brain and muscle, not a single wheel can turn;
We can break their haughty power, gain our freedom when we learn;
That the union makes us strong
In the United Kingdom, sympathy strikes were outlawed by the Trade Disputes and Trade Unions Act 1927 in the aftermath of the general strike. That was repealed by the Trade Disputes and Trade Unions Act 1946, passed by the postwar Labour Government.
Solidarity action remained legal until 1980, when the government of Margaret Thatcher passed the Employment Act 1980 to restrict it. That was followed by the Employment Act 1990, which outlawed solidarity action entirely. The laws outlawing solidarity strikes remain to this day.
In 2005, union leaders called for the legalization of solidarity strikes in the aftermath of the strike action against the catering company Gate Gourmet, but Labour ministers stated that they had no intention of repealing the law.
(Emphasis mine.)
Cowards, but what do we expect from neoliberals.