I think there are several points here.

First the protocol.  Think about tcp/ip, smtp, dns, ssh, ftp, http, https, ssl, pop, ntp, telnet etc.  How many developers at the protocol level are there?  How often and how many changes are there to the protocol itself?  Very few.  So from a core protocol level, the changes should be very few indeed.

Next an implementation of the protocol.  Bitcoin core/bitcoind is a reference implementation of the protocol, albeit an important one, but there are other implementations and other developers working in the bitcoin ecosystem.  If you look at the entire ecosystem and compare it to the Linux ecosystem, then things change.  There is a lot of money and there are a lot of developers working on various facets (e.g. colored coins, bitcoinj, bitcoinjs, exchanges, wallets, and many, many more).  I don't think you can compare the Linux kernel with bitcoin core, they are different beasts.  For example, you can run bitcoin without bitcoin core.  You can run Unix without a Linux kernel, but I'd say you couldn't run Linux without a Linux kernel, it would be something else.  Just because there are fewer developers working on bitcoin core, doesn't mean there are not enough developers working on the ecosystem itself.

:-)