Bcoz.. In meiosis ti change ploidy of cell from diploid(n) to haploid(2n) .. 2 division occur.. This helps in formation of haploid gametes.. When two gametes fuse.. They form diploid (2n) zygote.
On the other hand..
There is only one division in mitosis...bcoz no change in ploidy.. Use in growth. Bcoz it retains nucleo-cytoplasmic ratio.