One reason for this is that they did not grow to massive circulation until the late 1800s and early 1900s. Local and regional newspapers were widely read and covered both national and international news (in fact, early newspapers mostly covered international news).
We do realise that national newspapers can be very valuable for researchers and are looking at adding national titles to complement the local and regional titles we already have. We will start with newspapers covering the World War One period, but aim to add many more over time. Watch this space!