LTE is immature (handovers between LTE cells and especially to and from GSM and UMTS cells often fail), handsets are expensive, coverage is patchy and as dg7feq said devices need to fall back to GSM or UMTS for every voice call because the industry couldn't agree on a common standard for voice for ages. In addition there are over 30 frequency bands specified for LTE which will make global compatibility of handsets nearly impossible. Also as the following chart shows HSPA+ (if using the latest enhancements) provides similar and for on small frequency blocks even better spectral efficiency than LTE:
Also worth reading on this topic:
WirelessMoves: Why the US Needs LTE Smartphones in 2012 and Why They are Not Needed and Wanted in Europe