Mappers were likely using Potlatch 2 despite Flash at that point. For the first couple years, iD still had plenty of usability issues, especially around keyboard navigation, its presets and fields were still quite basic, and its performance was not at the level that mappers had come to expect from Potlatch 2. The problem was measurably worse in some popular browsers like Firefox and especially Internet Explorer. (Bet you didn’t expect to hear that name come up today.)
Rapid in a somewhat different spot these days. Parts of the application have been rewritten to perform significantly better than iD. Otherwise, it retains much of what mappers expect from iD, being a fork of it. On the other hand, some aspects like localization are still rougher around the edges, and lots of community members have formed opinions about Rapid without being regular users.
Apart from that, muscle memory would be a big factor dampening uptake. I think we may still have some very committed Merkaartor users.