It probably has to do with tax laws for non-profits.
Building a world headquarters is likely an allowable expense for charitable purposes. It is a place to put their money.
It will have nothing to do with religious matters or growth and everything to do with what they are allowed to spend their money on and not pay tax.
Oubliette: Actually, one of the GB, Guy Pierce, publicly admitted at the 127th annual meeting of the Watch Tower Society, held on October 1st, 2011, that they did NOT know if it is/was Jehovah's will or not to do this.
Yeah, they were probably waiting on a tax ruling.