As already noted, it’s a blood-brain barrier issue. The first generation antihistamines are about as ideal as you can get for penetrating the BBB - mostly non-polar molecules with a few amines. The second and subsequent generations took inspiration from first generation metabolites that also contained carboxylic acids. The additional functionality made them especially bad at getting through the BBB while retaining their activity at the target. It’s really a rather neat piece of medicinal chemistry.