Choice of measure for unemployment
What is the reason for the U3 rate being the more popularised and widely emphasised measure of unemployment in the United States and also for other developed economies? 


Historically, data for U-3 have been collected for longer than U-6.

For example, in the US, the former have been collected since 1948, while the latter only since 1994. So, it is "traditional" to report the U-3 data.

In many other countries, the equivalent of U-6 data may not even be collected.

So, when making comparisons across time and countries, U-3 data are the best we've got. 


  This broad measure is deemed by numerous economists to be the "true" unemployment rate.


This is incorrect. Economists do not consider any single measure to be the "true" unemployment rate. Instead, to arrive at any conclusion, economists try to look at a broad variety of measures and data, where available.

(But again, for reasons already given above about data availability, economists will often use only the U-3 data. This though should not be misconstrued as them thinking that this is the "true" unemployment rate. In fact, the deficiencies of the U-3 data is exactly why economists encourage collecting also other measures of unemployment.)