Because BITS is a premium institute which offers high flexibility in terms of academics. I know of a guy who took mechanical engineering in BITS but was interested in ECE hence he never attended any class of mechanical but attended the ECE classes and followed his interest and made use of the 0 attendance system. He studied mechanical for exams and profs passed him with the impression that he works hard for ECE. In the end he became an entrepreneur in his field.
Now PES is not a premium institute compared to BITS. PES is a money minded institution and behind its every action lies a financial gain. In 2014 it became a university free from VTU afliation and that allowed to conduct its own entrance exam known as PESSAT. It can control the fee structure for the PESSAT candidates and they are exorbitantly high and are more or less equal to that of BITS. The high fees is justifies if you take up computer science only. Because that’s what PES is good at. Due to shifting of job market towards IT, it realised its gain in increasing CSE and ECE intake. They train these students for placements specifically and then brag about getting higher salary packages than NIT karnataka. This is true but mostly applies to the elite students who slog their ass off. Average students and their parents are lured into thus by the immense amount of advertising it puts up during admission process.