'dat581' you should set the program priorities to suit your proclivities for sure - otherwise this is the reason (as seen in above quote):
"...The F-35B will also eventually be required to perform vertical takeoffs, but for now, the STOVL trials have been deemed more important..."
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ADDITIONAL QUOTE re VTO testing requirement:
F-35 Tests Proceed, Revealing F/A-18-Like Performance 16 May
2011 By DAVE MAJUMDAR
F-35 Tests Proceed, Revealing F/A-18-Like Performance | Defense News | defensenews.com
"...Eventually,
the F-35B will perform vertical takeoffs, but that testing has yet to be performed because other STOVL trials are of more immediate import, Kelly [Marine Lt. Col. Matthew Kelly, an F-35 test pilot with an F/A-18 Hornet background] said.
"There is a requirement for that and we do plan on
performing vertical takeoffs," he said...."