Nandi signifies strength, load bearing capacity and virility. Given that Shiva didn’t have to fight fierce battles or travel quickly between the worlds (he spent most time meditating) he didn’t have to choose for a more agile Vahana (Ride) similar to Vishnu’s. Given that bulls were the main form of transportation in rural India, it made perfect sense for Shiva – who is the most down to earth & connected with rural India – to take a form that was close to the people.