The latter were defensive struggles in contracting labour markets as workers struggled to hang on to their jobs and working conditions.
The former involved new layers of the working class - technical, administrative, and professional, many of them recent college and university graduates - confident in their ability to secure gains in their expanding fields.
Optimism in your collective bargaining power and the knowledge there is another job elsewhere if conditions don't improve in your workplace are the biggest contributors to labour militancy. That mood underlay both the rise of the industrial unions as economies recovered from depression in the latter part of the 30s, and the rise of the new service sector unions as the welfare state expanded in the 70s.
By contrast, the militant but ultimately doomed strikes in the 70s of air traffic controllers, teamsters, construction workers, miners, etc. unfolded in industries where there were labour surpluses rather than labour shortages - where unionized workers could be easily displaced by non-union workers available to perform the work at less pay and by tech change and foreign competition. The industrial unions have been so battered by these pressures over the past three decades that their mostly aging workers are now much more apt to push for buyouts than to desperately try to defend their jobs and conditions.
Not to be forgotten also in considering the factors underlying militancy is that there was a very strong left within the industrial working class in the 30s and a smaller but still influential left within the public sector unions in the 70s, including many 60s radicals fresh off the campuses. In each case, they helped channel spontaneous working class action.
The same dialectic was at work when militancy turned down. The disappearance of the working class socialist movement and of the student radicalization coincided with the preciptious decline of the industrial unions and the containment of the new surge of unionization to the public sector, where it had been easily won and, in some cases, freely granted by governments. At the same time, Western capitalism revived as a result of revolutionary advances in communications and transportation technology, and the anticapitalist bloc in the USSR and China collapsed. It is is very difficult to disentangle these developments from each other, and they cumulatively explain why the international left has become so marginalized.