At some point in time, some people accumulate more credit/reputation for their work, and can use that to earn more credit more quickly. This then snowballs as these people pull ahead of others out of proportion to their actual contributions.
This makes a Meritocracy an unstable equilibrium, and such a system will not remain meritocratic for long. When other parties come along later, they won't be able to displace the incumbents.
You might then be tempted to cut off the long tail and hand over the community to the incumbents. But that's really going to be a problem if more than half your work is being done in that long tail!
Importantly; if you do hand over to the incumbents, you've very obviously just changed into something other-than-meritocracy.
My argument is that all systems that start as a (naive) Meritocracy will tend to fall into this trap.