I think a lot of people find it easier to write rather than speak because writing can be less immediate. When we are speaking, if there are awkward pauses or things don't flow it can completely throw us off. Sitting and writing in a language gives our brain a chance to process stuff in a different way. I also think that's why some people can listen to a language and understand it, but respond in their mother tongue. For instance, my grandparents were born in Italy but my mom was born abroad. My grandparents spoke both Italian and English, but my mom would never speak Italian - she understood it 100% though because she would answer any question they asked her in Italian, just in English back to them!