Chasing new business in a more lucrative market is great. It energizes everybody when they see fast results. But why would you not serve the needs of your original customers just because they are not so profitable today? Tomorrow, the legislation changes and you find the old market is the new, profitable one and you have to start again with building (and repairing) important relationships.
Relationships that took a whole generation to build.
Look after your customers and they will look after you.