Because dear Rube with one's sixth ascension comes a 30% total overall stat bonus and the last of the 5% increments. Every ascension after the sixth brings with it only a 1% cumulative stat multiplier increase as opposed to the 5% that your receive with the prior six.
What most people have trouble with is the fact that the name stays the same for the fifth and sixth ascensions, that name being of course "Living God".
So RD in short you go to the sixth and not the fifth to make the most out of the last 5% stat bonus. Also, for those unaware, your sixth ascension also adds another 5% increase to your Ascended Blessing Tech bringing your chances up to 25%. Not sure if that infringes on the secrecy rules but suffice to say, if you know what the Ascended Blessing is then you're privy to the percentage information. To those that aren't, find a bootleg "Guide", I'm sure it's lurking around somewhere.
Regards,
The Deity
P.S. There is another reason to continue ascending after your sixth and that’s solely to boost your ascended account using APPs. The likes of Rhett have done this, not sure about the current master of the ascended server, one Mr Virgil. If you’re still curious, you could ask him. Just don't pester him too much as the result may be that you never see the ascended server...
P.P.S. Hey Xeno and Spacey

I hadn't seen you around for a while Mr S but you've probably posted a reason for your absence. I wish I had more time to trawl through the forums but alas I don't. With Lord Connery's blessing may this post fulfil my nostalgic “New-Comer Helping Days” requirement for a while.