When you compare the price per ton of pellets versus coal, you also have to consider energy density - you have to burn a greater weight of pellets than coal to produce the same amount of heat, so the pellet price is even higher than it first appears.
The actual density (weight per unit volume) of pellets is also much lower than that of coal, so you need more ships and rail cars to carry the fuel required. This was apparent when I worked at Lynemouth - the rail depot that used to handle the coal easily was stretched to breaking point and in need of expansion after the plant was converted to biomass. They also had to build extra silos to stockpile enough fuel to last between one shipment and the next - they like to stockpile at least a week's worth of fuel in case of supply problems, and a week's worth of pellets takes up a lot more room than a week's worth of coal.
There's also the energy input for producing the wood pellets to consider. The trees have to be felled, transported, cut up, and the pellets are then dried, compacted, etc. ready for transport. Much of the energy input to these processes comes from burning fossil fuels or from the electricity grid, but some estimates put the energy use of these processes equivalent to burning 10% to 20% of the wood in order to process the remaining 80% to 90%.
Of course, coal also has to be mined, broken up, washed and transported, but the energy inputs for those processes are generally much lower than for wood pellets. In the case of the UK, the power stations were often built directly over coal fields to minimize the transport costs, but now those same power stations are burning imported wood pellets, the cost of transport, both in energy and financial terms, is much greater.
The wood pellets also have health risks. The dust from the wood pellets is a known carcinogen, and special dust extractors, filters, and personal protective equipment are required at all stages of the transport and handling process.
The furnaces that raise the steam at Drax were designed for coal. Apparently the fumes caused by burning pellets contain extra acids or other chemicals that attack the steam tubes shortening their expected lifespan. One of the projects I was involved with at Drax, was to feed some sort of mineral powder additive into the furnaces that helped correct this problem. Of course, extra silos, monitoring and metering equipment was needed to store and feed the powder (I worked on the weighing equipment that measured and controlled the feed rate of the powder). The weight and volume of the powder was a tiny fraction compared to the pellets, but that powder still had to be bought, transported, stored, and monitored. I was never able to find out the actual composition of the powder - there seemed to be great secrecy concerning what it was. I don't know if that was for commercial or other reasons.