Probably because you're used to modern bars. I hope .
I found the same thing a while back after I built up a Kona with 25.4mm clamp X-Lite riser bars, straight away I noticed how flexy they felt compared to modern (and wider) 31.8 bars.
I now use Azonic O'Neal 25.4 riser bars and they still flex. I'm kind of used to it now, but I can appreciate the stiffness of the 31.8 bars when I ride my modern bike.
