I think that there were several reasons for Lutheran reaction to monasticism. The personal disappointment of Luther, after having suffered almost a physical and mental collapse in his attempt to reach holiness as a monk. The moral corruption he (and many others) witnessed in those days among the monks and clergy, the wealth and power of some monastries, but particularly Luther’s view of works.
It is a kind of oversimplification to say that we Lutherans reject the works. According to our doctrin - to put it simply - the works do not earn us salvation but the lack of them causes our damnation.
But Luther understood as works something that God has willed and commanded. The monastic piety as he saw it - and remember, he had been a monk - consisted of so many “works” that he afterwards looked at with contempt - pilgrimages, rituals, carrying medals, private mortifications etc.
To these Luther, in his Larger Chatechism, contrasted a little girl, who takes care of her small brother or sister obedient to the fourth commandment, and said that this is more than the whole monastic piety, because this is something the the Lord certainly wants and has ordered, but - according to Luther - not candles, bells, pilgrimages, and self-imposed mortifications (this is probably the origin of the story of a mother changing diapers as contrasted to monastic life).
While Luther’s view might look exaggerated, one also has to remember that both in west and east the monastic life had been put on such a high pedestal, that the people living in the “world” were regarded as second class Christians (I think that it was St. Basil who, while commenting on Christ’s parable on wheat, said that a monk or nun produces 100 grains, while a righteous layman produces 40 - and thought himself generous). It was the Reformation and , on the catholic side persons like St. Francis of Sales, who returned to the laymen and lay professions their Chrsitian dignity.
But not to give a totally one sided view of Luther’s opinion of individual monks (and faithful Catholics up to the end) one should mention that he always revered and respected his former abbot, Johann von Staupitz, because of his spiritual guidance and comforting common sense, when Luther’s spiritual depression and melancholy almost drove him insane (well you Catholics probably can drop the “almost” from the previous sentence).