Powerful and effective do not have to mean complex. Generally speaking, the smaller a library or tool is, the faster it performs and the easier it is to debug.
This trend is a rediscovery/reinvention of the old UNIX philosophy of do one thing, and do it well. Groupss of small, well written programs combined makes for a more customized end result. There are no extra, unused features that often just take up space in larger libraries and occasionally have side effects as the code is refactored or the library is updated.
As another person noted, it's also a reaction against frameworks like jQuery, Bootstrap, Angular, Polymer, and yes, even React. Some people prefer to combine only the functions they need, rather than include an all-in-one dependency.
For better or for worse, code size is one of the easiest metric to measure, and typically the first thing that will be targeted when a niche reaches a certain level of saturation.
In Javascript in particular, the micro trend is largely a pushback against major frameworks' tendency to be bloated (Angular, Ember and even React - given its scope)