I didn't understand by what you meant by 'just till Poona'.
Until 1983, there was no direct BG route from Bangalore to Bombay. In 1983, after the GC of the Bangalore-Guntakal route, the first direct BG route to Hyderabad and Bombay from Bangalore was available and trains were introduced straight away.
Udyan...
more... was among the first trains to be introduced after the GC of the Bangalore-Guntakal route. For the first few years, it ran combined with the Madras-Bombay Janata express between Guntakal and Bombay.
As to why it wasn't a dedicated train right from the beginning, you should understand that this was in the 80s. Madras was much larger and more important city than Bangalore at that time, and also the traffic to Bangalore wasn't much, as the IT boom was still over a decade away.So, the demand to Bangalore wasn't too much. Also, as I mentioned earlier, there were 3 MG trains to Miraj, all of which had connecting BG trains from Miraj to Bombay. So, effectively Bangalore had 4 daily trains to Bombay, which was more than enough.