According to some websites the prayers we can join, due to an excuse, are zuhr and asr or maghrib and isha. Can fajr be joint with anything and if not why?
The reason for joining prayer is to prevent difficulty while still maintaing the importance of specified time.
The shar‘i texts clearly indicate that it is obligatory to offer the five daily prayers at the specified times, and that it is not permissible to put together Zuhr and ‘Asr, and Maghrib and ‘Isha’, without any excuse such as sickness, travelling, rain and the like which would make it difficult to offer each prayer at its own time. Allah, may He be exalted, says
(interpretation of the meaning): “Verily, the prayer is enjoined on the believers at fixed hours” [an-Nisa’ 4:103], i.e., the prayer is obligatory at specific and known times. (See: Fath al-Baari by Ibn Rajab al-Hanbali, 3/7-8)
So here we join the two prayer which do not have great difference in their specified time. As for fard all other prayers time are far away so how it would be appropriate to join. It's important to pray at specified time as you can look into above mentioned ayah.
Remember cases in which prayers are joined and shortened in which of these cases.
May allah keep us steadfast.