Because the bulk of explorers are looking elsewhere.
Having said that, it is somewhat tricky to club the entire region as one. Sikkim near Siliguri corridor is extremely well explored. And so are a few places 500-700 Km east of the corridor (Guwahati, Kaziranga, Khasi Hills of Meghalaya). The ‘least explored part of India’ starts as you go deeper.
Depending on their motivation plus how they travel (and at the risk of generalizing), I categorize visitors in to three buckets for this argument: seekers (religious tourism), backpackers and portal tourists. In the context of north east India, they respond differently to the factors of safety (natural calamities, insurgencies, etc.), infrastructure (accessibility and rides), comfort (options to eat and drink, sleep, etc.) and securing a travel permit (for three out of eight states).
Seekers (religious tourism)
Seekers signify scale. Large numbers converge at a location in a relatively short span of time. Think of Kumbh Melas of Allahabad, Haridwar, Nasik, or Ujjain. These attract about 30 million visitors per day. On the other hand, the annual Ambubachi Mela (22-25 June 2016) at Kamakhya Temple (Guwahati, Assam) or the Deepawali Mela at Tripura Sundari Temple (Udaipur, Tripura), two of the biggest festivals in north east India, each manage 10 million overall. Most visitors are from either from the region itself or from the nearby states of West Bengal, Odisha, Bihar and Jharkhand. That is primarily where followers ofreside. .
Seekers form one of the most resilient group of travellers and are the harbingers of other category of travellers. Take for instance, how Gokarna, Kasol, Pushkar, Rishikesh or Varanasi have turned out as popular backpacking destinations. In north east India, only the key Shakti Peethas, Shiv Dol Temple in Assam, Satras of Majuli, and Buddhist monasteries have managed to attract decent number of visitors. Other places of worship remain relatively under the radar. Be it the ASI-managed site of Malinithan temple in Arunachal Pradesh, Nartiang Durga (Jayanti) temple in Meghalaya or Govindajee temple in Manipur. Besides, like in Goa and Kerala, Cathedrals and Churches of Assam, Meghalaya, Mizoram and Nagaland could offer rich experience to visitors. North east India is well placed to promote religious circuits.
Backpackers bring a brand. Think of Goa or Gokarna. They go local and spend more time per location. As rugged travellers, they may overlook infrastructure and comfort. They chase value, push the boundaries and provide economic stimulus to some of the most remote places. But a majority is yet to discover what north east India offers and may prefer Kasol over Gangtok. Many, who do their research and finally decide to explore the region, hit the hurdle of travel permits./ remain a sticky issue in parts of the region. Backpackers often travel spontaneously with flexible dates and such permits are restrictive.
North east India’s innate charms remain largely unnoticed. Take for instance the Vaishnavite Satras plus island life of Majuli or the Hornbill festival and hikes to Dzukou Valley, the spicy Manipuri eromba and easy walks in Moirang, the relics of Unakoti and Chhabimura in Tripura or a taste of Arunachali aphung at the Ziro Festival of Music (ZFM). Travel guides can do better to promote these value propositions.
Portal Tourists: Portals pack prosperity. Tourists signify a relatively high cash-spent to time-spent ratio. Often, these are (under 50 yo) couples, couples with kids or even, couples with elderly parents. But families are risk averse and value conscious. Even though only a handful of places in north east India suffer from insurgencies, the entire region gets stereotyped as unsafe..
Portal tourists have a significant spiralling impact on economy, resulting in local employment generation. This segment is a good target audience for the glorious festivals of north east (as hotel rents sky-rocket during this time). Be it Anthurium, Hornbill, Sangai or any other. Popular travel portals (like Yatra, MakeMyTrip, ClearTrip, etc) could better themselves to develop and offer credible packages for the region. To start with, they could make people aware that the places offered in the package are peaceful.
If tourism can flourish in Ladakh without all-weather roads and political unrest in the neighbouring Kashmir valley (both in the Indian state of Jammu and Kashmir), there is no reason why north east India, that is largely peaceful, should lag. It would help for stakeholders to address residual negative perceptions about security and consider relaxing travel permit requirements (ILPs). The hospitable people of the region make sure that you bring back the choicest of memories.