Lighter bicycle locks not made of steel Nowadays there are so many different high-tech materials, so I wonder if there is a suitable material for bicycle locks that would be much less heavy than hardened steel. If there isn't one, can somebody explain from the perspective of materials science why?

The only non-steel material used in locks that I could find is Kevlar, but mostly just as a coating around steel, and from various reviews I don't get the impression that they are better or lighter than ordinary steel ones. My understanding is that kevlar has good tensile strength (so gun bullets can't tear kevlar fabric) but it doesn't resist being sawn or cut. Carbon fibres would presumably have similar issues, it's fairly easy to damage carbon frames if you compress them so a potential carbon fibre lock wouldn't resist bolt cutters.

So are there any light materials that would have the correct properties for a bike lock? And if not, why not?
There isn't all that much point. 

We have a lot of knowledge on how to make steel tough. It's also pretty cheap. This basically makes it win-win for manufacturers: use enough tough steel in a good structure with a good locking mechanism to get a decent lock. It doesn't cost a lot and its easy to fabricate. 

The weight of many good locks is not that much to begin with; the main design goals with a U-lock is to leave as little space as possible to get a jack in, hard enough to cut, and has a proper lock mechanism. So, locks like the Kryptonite Evo Mini line are very good for many locking jobs and don't weigh all that much (about 1.5 lbs for the Mini 6). So, there's not all that much point in reducing weight. 

There are some locks which are not steel -- Tigr makes titanium locks, for example. But they're less well known and twice the price of a high quality steel lock, such the well tested Kryptonite New York Lock line. And the smaller units don't necessarily hold up.