I think support. You have to remember that the first Red Hat Enterprise Linux is from 2002. Back then, the old UNIX vendors were in steady decline, but the freshness of enterprise Linux probably made potential customers weary.

Nowadays, things are really different. Red Hat is large and has a solid track record. IBM has shown its support for Linux over an extensive period. SUSE is now in the hands of a huge IT company.

Consequently, I don't think there is a good excuse in 2014 not to seriously evaluate Linux. In fact, it should be (and often is) the standard option for embedded devices.