why is les mis all singing?

Isn't the musical sung all the way through anyhow? You saw what is known as a "concert" version. Sometimes the concert versions will leave out smaller bits of music or dialogue along with action. Sometimes the participants will be in costumes, and sometimes they won't. People who generally go to a concert version of a stage musical are usually pretty familiar with the storyline.

If you never saw the original musical, or saw any of the many film versions (include non-musical versions) or read the actual book--you might indeed be a bit confused from time to time. Those complaining that the musical is too long, never picked up a copy of Victor Hugo's novel on which the story is based.

The songs in the musical actually explain a lot of what is going on, so a copy of the libretto (the words in a musical or opera) would have been helpful if you find it difficult to understand all the lyrics while they are being sung.

being a musical it was to expected. you'd have to ask the producers of the 25th anniversary show why they did it like that. i was in that play years ago, we had a lot of fun running around and trying to sing without messing up.

It's a musical.