The answer is simple: it is in their financial interest to do so.
The Modern Critical Text (MCT) is an ever-changing uncertain text. It relies on archaeological discoveries, new interpretations of those discoveries, and constant updating of the text as a result of those discoveries and interpretations.
Therefore, it is unsurprising that unbelievers such as Aland (of the Nestle-Aland edition of the MCT) and Metzger (of Princeton Theological Seminary) have spent their entire career on the ‘scholarship’ of MCT. The ever-changing and uncertain nature of the MCT provides constant academic opportunities and a steady income resulting from those opportunities.
For example, by doubting the Pericope Adultarae (John 7:53-8:11), ‘scholars’ could write books and research about where and when they think these 12 verses originated. Another discovery of an ‘ancient’ manuscript would lead to research and discussion over if the MCT should be updated. Can you not see the constant availability of work?
If all Christians accept the fixed and authoritative Textus Receptus (TR), whole university and seminary departments would be shut down and many ‘scholars’ would need to find another job. Therefore, it is very clearly in the financial interest of those ‘scholars’ to heavily promote the MCT while aggressively casting doubts on the TR and unreasonably vilify TR supporters at the same time.
Dear reader, why should we trust the ‘scholars’ and the MCT when there is so clearly a conflict of interests?