On top of my head, I think the big difference is the popularity and support Ubuntu is getting. If one OS like Ubuntu is getting a lot of support and is backed by a good company (Canonical), there is a non-stop development and security updates being written for the said OS. Regular and continuous security updates makes is it "safer" than the rest of other linux distros.
No I would argue there are plenty of other fish that equal Ubuntu in terms of security.
For instance in openSUSE's case it is backed by Attachmate who owns Novell the former parent company to openSUSE and its commercial counterpart SUSE enterprise desktop.
The two main goals of SUSE in general is stability and security which trickle down to its non commercial counterpart..
Security upgrades come regularly and its usually openSUSE who gets the bugfixes first and those bugfixes trickle up.
Or how about Redhat and its non commercial counterpart Fedora, also have great focus on security though with fedora it can be a little messy on the stability front (well it is a little more bleeding edge which can be both a good and bad thing)
How about non commercial distros like Debian, Debian too prides itself on the merits of stability and security.
Plus a lot of the other distros such as Mageia, Pclinux, Arch, Gentoo and the grandfather distro Slackware all have fully dedicated teams who ensure a nice secure system.
Commercial enterprise does not equal security, if its a company that should have told you that its Microsoft.
After all here is a company that for years ignored major security issues and did not resolve them until many systems were compromised.
That is what happens when corporate interests get ahead of common sense, heck its a lesson that
Apple had to learn recently.
Now mind you I am not saying anything is perfect as linux too has its fair share of security issues but its not as common due to its standings in the mainstream.
Though to be honest I think linux would be a lot better off then both OSX and windows if it ever hit the big leagues due to the open source nature of the system.
Still though one must not give into falsehoods and say that no OS is bulletproof.