Cost of acquiring technology from TALGO is the main factor. Just owning/purchasing a few & very costly TALGO train sets shall only add to the variety of rakes and associated maintenence problems.
Since we are already paying heavily to acquire much better Shinkanshen technology as a part of MUM-AHM Bullet train project. It may be better, if we can make optimum use of this technology to improve the speed of conventional rakes, rather than paying extra for TALGO technology which is not as reputed/developed as Shinkanshen Technology.