It's not like they can't monetize this site, they just don't know what they're doing.
The way to monetize Slashdot isn't to drive out the existing and very knowledgeable users/contributers by bringing in a site that caters to the masses.. no, it's to use this crowd's high level of technical expertise/knowledge to make profit. How? Charge for ask Slashdot! Have a technical problem and need assistance, and stackoverflow isn't cutting it? Well shit, pay Slashdot and get an article posted and bam, your problem will be answered by a slew of very intelligent people.
The other thing is jobs. I know Slashdot has a few job listings or whatever, but they aren't doing it right. They have a massive pool of people, some of whom are unemployed (if you read the comments). Why not hook up contractors/head hunters with these people through the site, for a price? Why not open up the subscriber base with an OPT-IN (by default opt-out) option where potential employers can contact us, and even post jobs (properly)?
These are two very simple ways they could monetize, and I don't have an MBA. I'm sure there's many more. Sometimes it's not about straight up ad-click and growing page views, it's about being intelligent, and working with the extremely valuable resource you have: very smart people who give articles and discussions for free!