The default partitioning of CAM space on Cisco gear is the obvious issue, but the root cause is the massive deaggregation of announced IPv4 routes on the Internet. You can see various statistics about this problem at http://www.cidr-report.org/, but the short of it is that if the top 30 networks (based on announced route savings) completely aggregated their announcements as much as possible, ~41,000 routes (~8% of the routing table) would be eliminated.

And that's just the top 30 networks — if every network cleaned up their announcements, it would eliminate ~232,000 routes (~45% of the table).

Adding to the deaggregation problem is the inability to easily filter out route announcements based on RIR minimum allocations without having to add tons of exceptions for CDNs that operate as islands of connectivity and carve out IP space for each island from a single address space allocation. (There's no covering route for the islands of connectivity since these CDNs have no "backbone" connecting the islands, so if you filter out those smaller announcements, you lose connectivity to those islands.)

There are many people who think this problem will just magically go away as IPv6 adoption increases, but all increased IPv6 adoption will do is make limited CAM space even more limited as network engineers have to balance dividing precious CAM space between a ballooning-quickly IPv4 route table and a ballooning-slightly-less-quickly IPv6 route table.

(To be clear: I think ubiquitous, functioning, end-to-end native IPv6 connectivity needs to happen sooner than later, but it's not a magic bullet for the Internet's technical problems.)