Because you haven't learnt any of the corresponding level 3 deathblows outside of your gear.
This Gamefaqs post shows you which non-gear deathblows correspond to gear deathblows: http://www.gamefaqs.com/boards/199365-xenogears/54954082
Having played the original back in the day and replaying on a PS3 recently, I was initially pretty distraught at how ugly the game looked on modern HD systems. I ended up buying one of these thingies for $35 from Amazon, it's a little box with HDMI in/out and generates a scanline effect and has a couple adjustments. That's all it does.
And I gotta say, it makes these old 2D games look a lot better. It's not a perfect approximation of playing on a CRT, but it's preferable to most any other option I have -- outside of emulating and just running a computer into the tv. You should look into it, it's made my experience a lot more enjoyable playing PS1 titles on the PS3.
I’ve also never heard of it but googled it for us both, looks like it is a sort of album book containing the entire main script as well as screenshots and other Xenogears art.
https://www.amazon.com/Xenogears-Thousands-of-Daggers/dp/4925075276