a giant head start, brand, trade secrets (hardware and software), strategic supplier & manufacturer relationships, strategic partnerships (app developers & carriers)
Being the "first mover" is more often than not a disadvantage, and being one entails taking huge risks. ("Copycats" by Oded Shenkar is supposed to be a good book about this. Google "first mover fast follower" and "copycat innovation" to find additional relevant references. )