I can think of a number of reasons.
Firstly, what does history teach? It demonstrates that science has filled the gaps with explainations based on logical and rational observations of the world around us. Science has never filled a gap in with anything that requires the supernatural intervention of a divine being. Logic therefore suggests that where we have gaps now then these will be filled in with answers that further reduce the space where a beleiver insists God might needs to be involved.
Secondly, science often predicts what will be in the gaps. Think of the periodic table. Think of the Higgs Bosun. The existince of elements that could not be proven experimentally were proposed by those whi could see where the chemical order led. The Higgs Bosun was predicted to exist and then proven to exist. No temporary act of God or nod to God required.
Thirdly, it's yet another area where JWs are trained to cop out. All my life I was trained to cop out of things:
- education: let others be the doctors, dentists, physiotherapists, lawyers, engineers, scientists. Who needs them when we have magazines to push on a Saturday am. Except when we need them to treat our wrecked backs from 40 years of window cleaning, negotiate multi-million pound property deals, develop all the technology that allows us to live just like everyone else.
- public service: don't think about being in the fire, police, ambulance, nursing service. Think of the meetings you'll miss with shift work. I know some do but it's not exactly common amongst Witnesses.
- charity works. Give a bit of money if it makes you feel better but don't waste your time travelling to volunteer in a famine ravaged nation or risk your life working to help civilians caught up in civil wars
- blood. You want to donate blood? Really? You'll be thrown out. Nothing could be more wrong. Except when pretty much every dub is quite happy to grab all the fractions the WTS says are OK that can only come from processed, donated blood.
As a religion, Witnesses are takers. Consumers but never producers. Leeches on the hard work of everyone else who then look down their collective nose at them because they don't beleive the same thing.
The God of the Gaps is just another cop out. Just another way of thumbing our nose at the work and dedication of those who are seeking to find the answers through logic, reason and experimentation. It's just another way of saying to all the scientists of the world "Thanks for all your hard work in bringing us answers that allow humanity to develop, but since you can't easily combine GR and QM you are a bunch of losers who just need to quit and praise God".
Science will never have ALL the answers. But it will never stop progressing and it will never stop explaining things that the faithful just want to park to one side with a "it woz God wot dun it."