1 ) You have to buy installatron script for installing scripts like Fantastico
2) Not many are supporting direct admin servers like Cpanel servers
3) Few software's pre installed in Cpanel will not come with DA you have to install it
but this is very few to name, overall the resource usage of DA is too low compare to Cpanel so this runs fast and allows your server to run fast
I have to admit - $20 per server should not make a huge differance to the cost of shared/reseller hosting on that server, spread over 80 clients, thats only $.25 per client.
People (Consumers) like "brand power". Just like in the shops, people would rather by a well known branded item, than a lesser known brand, even though the well known brand is more expensive.
Thats why for the time being, DirectAdmin will not do as well as cPanel.