There are many discussions on the web about which one is better, and why one is more popular with a certain group of users than the other. Here is what I think why WordPress is more popular among the causal blogger than MT.
Note #1: I think MovableType is, in general, a better blogging system (e.g. more scalable, more flexible architecture, etc.) but oftentimes technical merits do not count for much when causal users evaluate software.
Note #2: I am comfortable in unix shell so that's not where I burnt my time on.
Fact: I've spent around 4 hours trying and failed to install the infrastructure (CGI and Perl's DBI and DBI::mysql or DBI::sqlite) on OS X to get MovableType to run.
Fact: I only need to spend around 30 minutes to get WordPress to run.