I think it's more of a supply vs demand issue, there hasn't been much demand for it so why put the time into converting it to a Mac version.
From what I've read Filemaker Pro is an excellent tool, been looking to get a copy and try it out myself.

If you do wish to use Access on the Mac there are a number of ways this can be done.
The way I do it is using Parallels which works really well, that's how I've been making the tips videos as I've got a program for the Mac called Screenflow for screen recording.
Others include
VMWare
VirtualBox

Stack Social has a good deal on it at the moment, it's worth keeping an eye out for sites like that and
App Buzzer
Bundle Hunt
...

Another option is using the built in functionality in a Mac called Boot Camp.

There are a number of iOS apps that work with Microsoft files for Word/Excel etc.
One I found that works with Access Databases is Impathic

There was also a rumor that MS Office was coming to the iPad. I doubt it will have Access but here's hoping.

Alex