The calendars will differ. Because the sun is rising at different timings in different regions. Calendars are actually a part of culture than based on a religion. Those are based on mainly the Sun. Most of the Hindu rituals are related to the nature and weather conditions. So according to the regions, these factors differs. Since Sanathana Dharma is the culture of India, the different calendars in Hinduism depend on the weather and nature conditions of India.
Most calendars will have mentions of the start of the agricultural season. Just consider the monsoon, the starting and duration of monsoon varies in different states, according to that there might be differences in calendars in South India and North India. This culture is very much related to agriculture, and it is a wide spread culture in a large country. There can be some variations in the same culture which will be most probably based on the nature of that region. So the festivals of specific regions also depend on this. Most of the festivals in India will be in the reaping season of the agriculture, so those festivals will vary according to states.
For example in Kerala, there is Malayalam calendar starting at the month of 'Chingam' and it is the reaping season. So one of the most celebrated festival in Kerala, 'Onam' is in the month of Chingam. So from states to states festivals may differ. There will be common festivals, but most of these festivals and rituals depend on the weather and nature. Even if there are some common festivals, each of them will have it's unique variations.
Each Smrithi is followed in a region for a particular period according to it's nature and weather conditions. Smrithies are allowed to edit accordingly whenever needed. So the experts in particular region could have edited them according to the conditions. This is the way our ancient rishis and experts defined it and it should not be changed unless there is variations in nature conditions.