Who are the "Foremost" in Surah Al-Waqiah and why so few of them from "later times"?

In Surah Al-Waqiah people are divided into three categories: *Quran 56:7-56:14:* >وَكُنتُمْ أَزْوَاجًا ثَلَاثَةً > > And ye shall be sorted out into three classes: > > فَأَصْحَابُ الْمَيْمَنَةِ مَا أَصْحَابُ الْمَيْمَنَةِ > > Then (there will be) the Companions of the Right Hand;- What will be the Companions of the Right Hand? > > وَأَصْحَابُ الْمَشْأَمَةِ مَا أَصْحَابُ الْمَشْأَمَةِ > And the Companions of the Left Hand,- what will be the Companions of the Left Hand? > > > وَالسَّابِقُونَ السَّابِقُونَ > > And those **Foremost** (in Faith) will be Foremost (in the Hereafter). > > > أُولَٰئِكَ الْمُقَرَّبُونَ > > These will be those Nearest to Allah: > > > > فِي جَنَّاتِ النَّعِيمِ > > In Gardens of Bliss: > > > > ثُلَّةٌ مِّنَ الْأَوَّلِينَ > > A number of people from those of old, > > > وَقَلِيلٌ مِّنَ الْآخِرِينَ > > And a few from those of **later times**. As far as I know, *later times* means after the Prophet was sent. So, who are these "foremost" and why they are few in later times?