In short: those distributions that do not package Apache OpenOffice are lacking volunteers that will do the packaging.
The long answer to this question probably can not answered to everyone's satisfaction. Depending on your Linux distro, either the administrative body for that distribution determines this, or the volunteers which contribute to your distro's repository make these decisions. So, in some cases, Apache OpenOffice has little input in this decision. But, if you are using a distribution which either allows for more user input, or maintains "community" repositories, you can effect this decision, either by packaging Apache OpenOffice for your Linux community repository, or by actively participating with your Linux community to help make this packaging happen.
For our part, Apache OpenOffice recognizes the need for more interaction with the Linux community and we will continue our communication with a number of distributions to make finding OpenOffice in distribution repositories a reality. Currently, we feel utilizing "community" repositories, rather than the formal repositories for distributions, in the best way to achieve this goal in the short term.
Comment: If some people want to collaborate bringing Apache OpenOffice to Debian I will help to do so. I myself have not enough experience to do it, esp. in compiling. (Mechtilde).
And finally, if you love your Linux distro, but can't assist with packaging, the best way you can help to bring OpenOffice to your chosen distribution is get involved with your distro's community through mailing lists or fora and suggest that OpenOffice be included.