Thank you for your question. With many of the issues in the sections on worship in Islamic law we do not necessarily understand the logic of the rulings as Islamic law is not derived that way (through independent rational speculation). It is derived from an examination of the scriptural sources according to methods that are discussed extensively in the science of the principles of jurisprudence. Sometimes, we can have an idea of the wisdom behind certain rulings, but in many cases, our actions are based on the purport of those sources.
This is one such case in which the logic is not known, but it could be rationally inferred that perhaps the wisdom behind it is that since the time of travel is earlier, and since God doesn't want the traveller to go through too much difficulty, they are required to break their fast, whereas if a person travels after zuhur the time till breaking fast is less and so it is not so difficult for them to complete it and therefore they are required to do so.
May you always be successful