India is not the only subcontinent.

In specialist literature (economic, political science, history and anthropology lit) of Africa, the Sub-Saharan region is often and uncontroversially referred to as a subcontinent, despite the fact that the region makes up approximately 80% of the continents total landmass.

Ditto, Mesoamerica (the pre-columbian region including much of modern Mexico and Central America) can and has been referred to as a subcontinent in specialist literature.

Of course, you are correct to note that the phrase "Indian subcontinent" is more ubiquitous than the phrases "Sub-saharan African subcontinent" or "Mesoamerican subcontinent" or even "Southeast Asian subcontinent".

I suspect that the frequency of the subcontinent label comes from effort at linguistic clarity. As /u/AnakinAni noted, several other countries like Pakistan, Bangladesh, Bhutan, Sri Lanka make up the continent in addition to the country of India. Thus, referring to "Indian subcontinent" is a way of clarifying that the author is speaking of the larger region and not the specific national boundaries of India the country. On the other hand, there is no state called "Sub-Saharan Africa" or "Southeast Asia", and so there is not a similar need to clarify that you are speaking about a subcontinent/geographic region. It can be generally understood from those names alone.

I'd also point out that some scholars prefer to use the term South Asia or South Asian subcontinent rather than Indian subcontinent, to make clear that they are talking about the broader region, and to avoid the idea of the 20th century Republic of India as equivalent to the Subcontinent.

India is not the only part of Indian subcontinent, but other SAARC (South Asian Association for Regional Cooperation) countries - Sri Lanka, Pakistan, Bhutan, Nepal, Bangladesh and Maldives - are also a part of the Indian subcontinent. This group, when combined together, form the Indian subcontinent. These regions were initially part of India, but were separated because of political or geographical reasons.

As a follow up, has subcontinent been historically used to describe Europe?

The climatic conditions of each of these seven countries match because they are joined together geographically. The other continents of Asia have different geographical and climatic features. Afghanistan is also a part of the Indian Subcontinent. The region might also include Aksai Chin's disputed territory. This territory was once a part of Jammu and Kashmir's British Indian princely state, but is Xinjiang's Chinese autonomous regions' administered part

The Indian subcontinent is one of the regions of Asia, which is located on the Indian tectonic plate.

'South Asia' and 'Indian subcontinent' are used in an interchangeable manner. South Asia's religions suggest that South Asia and Indian Subcontinent is one and the same thing. It is only because of political sensitivities that some call it 'South Asian Subcontinent' or 'South Asia', some term it 'Indo-Pak Subcontinent' and some refer to it as 'Indian subcontinent'.

However, when someone uses the Indian subcontinent for South Asia, small island countries of Maldives and Sri Lanka might not be included, and Nepal and Tibet might be or intermittently excluded, depending on what context they are mentioned.

As far as the geography is concerned, Indian subcontinent is south-central Asia's peninsular area. It is mostly is placed on the Indian Plate, separated from Asia. The Indian Plate has most of South Asia, extended from the Himalayan region to the Indian Ocean, which also includes some areas of Eastern Indonesia, Southern China, Karakoram and Kuen Lun ranges. However, this doesn't include Kohistan, Ladakh, Balochistan or the range of Hindu Kush. The Indian Subcontinent - with countries like India, Bangladesh, Pakistan, Nepal, Sri Lanka, Maldives and Bhutan - covers an area of about 4.4 million kilometres. This area is Asia's 10% or the world's 2.4% of the land.