Yoyo games took 2 years between announcing game maker studio 2 and releasing its open beta and now selling it.
They rewrote their engine too.
I think the reason construct3 is taking a lot of time is simple - Scirra is a much much much smaller team, with mainly 
Ashley working on the version 3, while also maintaining the current one
in contrast yoyogames releases native exporters, html5 exporters and has a bigger developer team.
They even released a plethora of learning videos with the beta:
http://www.yoyogames.com/learn2
They were acquired by a company with a lot of capital too. Yoyo games is based in Scotland btw.
The other competitor- clickteam now has a dev blog to wet the appetite and engage with people to get them invested
http://www.clickteam.com/fusion-3-development-blog
Both engines already showing up with brand new spanking features that address old weaknesses and also promise to be multi-platform in terms of editor and export (native AND html5) - construct3 is going to see some very stiff competition by the time it is revealed- especially now that people can start to learn about what's in the other ones and even download and use one of the two.
If clickteam releases a beta before construct3 even has a screenshot or estimated release date - it might attract a lot of the people hungry for updates on that construct3 website that is getting no updates whatsoever <img src="{SMILIES_PATH}/icon_lol.gif" alt=":lol:" title="Laughing">