Even i get this question a lot of times. Once i just picked up the pen and did a little bit of research on the same. Steem Blockchain dApps are not found to be selling advertisements currently. There could be two prominent reasons
They are getting Steem power delegations to generate enough income out of the curation rewards, which are probably bigger than the income generated by way of advertisements
On order to advertise you need enough visitors coming over to your website and these dApps are probably in the phase where they need to be focussed on community building to a certain level, after reaching a benchmark number of users they can start accepting advertisements.
I analysed the alexa data of some of the most popular dApps and figured out that the rankings of these dApps are as follows
Steemhunt ranks 130870 on 6.11.2018
Musing ranks 448221 on 6.11.2018
d.tube ranks 29533 on 6.11.2018
Why are steem blockchain sites not selling ad space? | Ecency
