On a different level, the answer is the same for many other questions:
- Why have a no-blood policy?
- Why insist on shunning DA/DF family members?
- Why keturn in monthly reports of field service?
- Why keep track of all partakers at the annual memorial?
- Why reject all holiday celebrations, both religious and secular?
- Why refuse to serve and honor your home country?
- Why should school children spend their summer vacations being "special pioneers" instead of playing baseball or soccer or climbing trees?
- Why forbid attending religious weddings, funerals, Bar/Bat Mitzvahs, birthdays, quinceaneras, etc. for friends and family members of other faiths?
A: The Watchtower uses all these as loyalty tests, to prove that they--a corporation in Brooklyn, New York--matter more to you than family, friends, country, even your own life!
If you are not willing to sit still for three days, listening to the same indoctrination you've heard before, spending the family's annual vacation budget to stay in an approved hotel and buy approved food, then you are not putting the Watchtower first. It's the first unwritten Watchtower commandment, the first commandment of any narcissist for that matter:
I. We are thy Lords the Watchtower. Thou shalt not put anything before us!