You ask what are the main reasons for the teacher's low professional motivation.
Let me start by saying that not all teachers have a low professional motivation. Though I am a University professor, I know of many teachers who are highly committed to their profession, and hence, are strongly professionally motivated as teachers.
Of course, there are now at primary and secondary schools many teachers with a low professional motivation. As I see it, that low motivation has to do, among other things, with the following:
1) In almost, if not all countries, the number of pupils attending school is, fortunately, increasing a lot nowadays. I say fortunately because I think that only education can save societies and countries from possible collapse, be it violent or gradual. Of course, education is costly, but it is much less costly than its alternative, ignorance.
2) Needless to say, when the number of pupils in schools was relatively low, they generally came from families whose educational, cultural, and socio-economic background was relatively high, what it is not the case nowadays. Pupils coming from, say, cultivated families are more motivated to learn and attend school than those who come from, say, relatively uncultivated families and backgrounds. Of course, motivated pupils are more likely to motivate their teachers than their unmotivated counterparts. This shows that teachers' professional motivation has to be seen as a bidirectional process, that is, pupils' motivation to learn increases teachers' motivation to teach, and teachers' motivation to teach increases pupils' motivation to learn.
3) With, say, the globalization and democratization of education and schools, teachers' social status significantly decreased. Because of this, teachers' professional motivation also decreased.
4). In a nutshell, the appearance of a global world, the democratization of schools, the attendance of schools by many unmotivated pupils, the decrease of teachers' social status are reasons for a low professional motivation among many teachers. As I said before, we have nowadays in many schools teachers who are highly motivated and others who are poorly motivated. This relatively paradoxical existence seems to be a characteristic of the world where we live. That is, more than ever, there are now manifestations of the true, the good, and the beautiful along with manifestations of the false, the immoral and unjust, and the ugly. This only compounds the idea of the coexistence of low and high motivated teachers and pupils in schools of today.
I hope I have got your question, and that this helps,