Exalted which is also called "Uchcha" (meaning Higher) in Sanskrit is an environment where a planet acts "its best" as per its "natural tendencies".
Key terms here are 1. Natural tendency of planet and 2. Best or Higher behavior of planet.
What is Natural Tendency of Venus - It is THE Planet of Love and all things related.
What is Best Possible Love? - Unconditional, without expecting anything in return (physical or spiritual favor). A love which finds more expression in platonic form rather than physical.
Pisces is the sign of Infinity. The last sign of zodiac which shows emancipation. The lord Jupiter which is representative of purity, virtues and higher thoughts. Also the very element "water" is most consistent with the natural tendency of Venus i.e. emotions.
Therefore Venus ends up showing its highest virtues depending upon its basic nature in Pisces.
Now a quick look at why other signs may not be attributed as Exaltation of Venus -
1. Sagittarius - This is another sign ruled by Jupiter and does direct Venus love towards spirituality. However it is fiery sign and Venus loses its basic nature of emotional love and becomes a bit practical and philosophical.
2. Aries and Leo - The other 2 Fire signs are also against the emotional nature of Venus. Aries is also ruled by Mars and a lot of passion is infused in this Venus. As for Leo, Venus is great here but can be a show off.
3. Capricorn, Virgo, Taurus - Most of the Earth signs can be ruled out because they make the person more sensuous. Including Taurus which is own sign of Venus i.e. Venus shows senses and all things sensual but it is not exalted in it.
4. Other Watery signs Cancer and Scorpio - Watery signs are not bad for Venus energy. Cancer much like Pisces is great for Venus however Cancer is very emotional and show lot more fluctuations in love. Scorpio - It is sign lorded by Mars and shows a lot of passion, so not exalted for Venus.