Reason No. 1
It is not developed by native English speaking people. This is biggest drawback of Drupal which is reflected in almost all aspect of Drupal for which I don’t like it.
Few years back when I first started exploring Drupal I gave up as I could not understand the way it was written.
On the contrary it just took me 10 minutes to setup my first Joomla based web site.
Reason No. 2
It is too complex. I’ve worked with almost all of open source packages and it is undoubtedly the most complex CMS package to work with
Reason No. 3
The contents are not organized as it should have been. They got nodes then they added taxonomy and topped up with terms, vocabulary etc and all these hotch potch takes all your time to figure out what to put where.
Reason No. 4 – Ecommerce
They are trying desperately to integrate ecommerce package into CMS. I appreciate the effort and see it is almost coming good, however the basic framework of Drupal itself is not organized and adding ecommerce further makes it complex to glues everything together.
Reason No. 5 – SEO
Begin SEO guru I always look for an easy way to enable SEO freidnly URLS, keyword etc but Drupal gives me sleepless night in getting it together.
Reason No. 6
It is slow. It takes time to load after mouse click, it really takes a lot of time.