To me, 'professional' is not necessarily a term of approbation. I'm not sure whether teachers have in any case been 'professionals' in any other sense than that they have (mostly) had to obtain some sort of qualification in order to be employed, or at any rate to be able to get a reasonably well-paid job. The same is true of e.g. plumbers and electricians.

I think the change has been in the status of teachers. The highest aspiration for respectable working-class families used to be that their child should become a teacher. Teachers were looked up to in the same way as doctors and clergy. All this has changed, for a variety of reasons. Teachers feel under-appreciated and under-valued. So they are more likely to do things like going on strike if they feel that injury is being added to insult. They have got less to lose.

(I speak as the son of two teachers and as someone who - briefly, unhappily and incompetently - was a teacher himself).