The first reason is time: It took about a decade to develop the B-2.

CENTERFORSECURITYPOLICY: Center for Security Policy | SUMMARY OF THE CENTER FOR SECURITY POLICY��S SECOND HIGH-LEVEL ROUNDTABLE ON THE FUTURE OF THE MANNED BOMBER FORCE

Overnight camping at Caracol will be allowed for the first time, allowing winter solstice campers the chance to witness the sun rise on a new B'ak'tun (or cycle) from a very historical spot.

BBC: Maya festivals for the end of time