I believe the lower number of audience of cricket as a sports is due to soo many formats and its hard for a new-viewer or potential viewership to contemplate all these changes in each format and jargon used in cricket.
I think the negligence of T20 is due to belittlement of T20 cricket by former or past cricketers, which makes audience and the current cricket setup to think it as of a lesser format or not "true cricket". Example -I have heard same ex-cricketers criticizing T20 who were the part of Kerry Packer league who also left their national team to play in non recognized league for money now the same ex-players are saying that the money and glamour is hurting/harming the game and is not "true cricket", was that not the same case when they joined the Kerry Packer league.
To get new viewership the game should be less complicated for people to understand and follow, I too like Test cricket but tbh this is not same for everyone and more particularly for newer audience.
There are so many variation across all different format for a non follower to understand which makes it less appealing to newer audience.
If ICC wants to improve on the current state and declining popularity of the game then they should be making the game less complex, may be say good-bye to 50 overs games. And take T20 with more respect and be seriousness.
After all; the sports are for entertainment they should be left that way rather over complicating the things. ICC and local boards should work with local schools to promote inter school Tennis/tape ball T20 leagues in various countries. Having 1-2 odd matches or a league in places like North America will not grasp the new audience. Because in those places the crowd that is turning up to watch are the people from countries where cricket is already popular and this will not serve any purpose.
Having Tennis/tape ball tournaments will improve the knowledge and interest of the local public in say "non-cricketing" Nations.