Recent state regulations have tended to cut back, rather than increase, time for in-service training. Additionally, schools have training requirements coming from many other constituencies, such as new mathematics and language arts standards, new standardized testing, and so on. Although those of us with industry backgrounds view professional development as a personal investment, necessary for survival in a competitive job market, if you consider the unacceptably low compensation of teachers, it is hard to blame them for not adopting this attitude.
Union membership tends to reinforce their position.
Hence, the gap continues.