The expense and disruption to dev cycles makes this impractical. They don't make money directly from showcases and if the products included show poorly it can hurt their sales in the long-term.
The reality is that its a delicate balance, especially now that digital distribution is dominant is it really desirable to promote in an environment/format where announcements can get lost in the mix? Why not pick a time where audience attention is less divided? Its not like people only watch trailers on certain days of the year!
After a year and a half since the last one they had almost nothing to show outside of CG trailers for GAAS crap.
The content if they made one every 6 months would be even poorer.