OP
It's probably for lots of reasons... but I know that tracking owners for stolen property is a very time consuming process. I heard of a discovery of several hundred stolen bicycles... many with serial numbers and security marking alegedly to enable tracing if stolen... Luckily one of the coppers took it upon himself to trace the owners... but it was like 2 a week he could shift ! I think in the end they mostly went to auction. The police station yard was full of 'em !!
I was was working in Wandsworth some years ago and there was a big police auction place where you could view and place sealed bids on loads of stolen goods from bikes, TVs , antiques , to power tools , to aftervshaves / perfumes , to mobile phones and electrical goods cds DVDs etc etc
you name it it was for sale.
Some of the stuff was quite interesting but showed how much was being nicked and never made its way back to the original owners