Let’ try to understand why Mercury gets debilitated in Pisces sign and in Uttara-Bhadrapada Nakshatra. Let’s see what these energies represent –

Mercury - Mercury represents communication skills, marketing, calculative ability, younger siblings, hobbies, skills with hands, intelligence, quick decision making, logical thinking, youthfulness etc.

Uttara Bhadrapada Nakshatra - It is a nakshatra related with next stage of Purva Bhadrapada. Hence, it is nakshatra of the actual event of liberation or enlightenment. It also represents all type of mystical subjects, practices and experiments. Please check this link for more info on Uttara Bhadrapada Nakshatra - https://www.astrosaxena.com/ubnaksha.

Pisces - As Uttara Bhadrapada is a part of Pisces sign, Pisces and things represented by Pisces are also important here. As Pisces is the 12th sign of zodiac belt, it represents the same energy and things which are represented by 12th house such as Isolated Places, Spirituality, Imagination, Other Dimension or Other World etc. Besides these things, it also represents your Hidden Talents, Ability to work in background, Spiritual Self etc. Pisces is consisted of last 2 and half nakshatra, i.e. Purva-Bhadrapada, Uttara-Bhadrapada & Revati. Pisces' lord is Jupiter.

Please go through the introductory article 

- https://www.astrosaxena.com/exdeb .

Reasons behind debilitation of Mercury in Pisces – Mercury represents intellect and it doesn’t do well in any of the water signs; i.e. Cancer, Scorpio & Pisces. It is because intellect doesn’t find a solid base to work on. It is floating intellect. This person cannot take practical and rational decisions of his life. This person cannot convey his thoughts effectively. As Pisces is sign of imagination and dream-world, intellect of Mercury doesn’t find the best of environment to work itself as this person’s intellect will always be working in imaginative dream-world. As water signs represent emotions, their decision-making would largely base on emotional response to the situations in life rather than practical or logical reasoning.

Hence, Mercury is debilitated in Pisces.

Reasons behind debilitation of Mercury in Uttara-Bhadrapada Nakshatra – Pisces is again consisted of 2.5 Nakshatras, then how Mercury is exactly debilitated in Uttara-Bhadrapada Nakshatra?

It is because this Nakshatra represents our higher spiritual experience. Experiences like Kundalini/Chakra Activation or Out of Body experience or final experience of awakening/enlightenment. Mercury is intellect and communication. Now, when we go through these higher spiritual or divine experiences, first of all your intellect cannot comprehend the event. You are unable to understand what is going on when you have an out of body experience? Then it is equally difficult to explain or communicate it to others in right word. Even those people who claimed to be enlightened or awakened in their lifetime, couldn’t explain that event to others perfectly as to what really happened or took place in that particular place. That’s where it is said that communicating about spiritual or mystical events is like a dumb person trying to explain how sugar tastes? There is no way to communicate for him. Likewise, there is no way to communicate a spiritual experience. Hence, Mercury is debilitated in Uttara-Bhadrapada Nakshatra.

Reasons behind debilitation of Mercury at 15 degrees – Again, it is the same logic. Mid-portion of any sign has its most impact or intensity. As we know that Mercury is exalted at 15 degree of Virgo as it is strongest there. Hence, it will be weakest or debilitated at 15 degree of Pisces as it is exactly opposite to its strongest position. Another way of understanding this is that person’s intellect is at its weakest when Mercury is at 15 degree of Pisces.