Answer:
Due to large size of genome.
Step-by-step explanation:
The study of wheat genome is difficult task because of its large size. The genome of wheat comprise of 15 billion DNA bases. The wheat genome is three times bigger in size as compared to the genome of mammoth. Mammoth is an extincted specie of elephant which were the biggest land animal at that time. So due to large genome size of wheat, the scientists takes more time to complete its sequencing (order of nucleotide in the DNA).