Why recurrent plant is always used as a female parent in backcross breeding programmes?

Answer:

The basic objective of back cross breeding program is to transfer one or two genes from a donar plant to a well adopted recurrent cultivar. Here the term 'adopted' means, adaptation of the cultivar to the wide environmental conditions. The back cross process aims to recover as much of recurrent parent genome is subsequent back crosses in addition to the donar genes of interest. 

There are two reason for using the recurrent cultivar as female parent. First reason is the recurrent parent factor. The fertilization process involves fusion of pollen nucleus and ovule nucleus. Pollens carries only the nucleus from the pollen mother cell. The the cytoplasm of pollen cells will be degraded during the maturity of the pollens. On the other hand, the ovule cells carries entire cytoplasm along with nucleus of the ovule mother cells. During zygote formation, pollen nucleus (n) and ovule nucleus (n) fuses and develops into zygote (2n) but the cytoplasmic content of female is remain undisturbed. The developing zygote thus carries fusion nucleus and female cytoplasm. The cytoplasmic organelles, chloroplast and mitochondria carries DNA which codes for genes involves in photosynthesis and transpiration. These vital process are directly linked with the yield and adaptation of a genotype to a particular environment. For this reason,  the cytoplasm of recurrent parent should be favored over donar in a cross. This is achieved by using the recurrent plant as the female, due to maternal  inheritance of cytoplasm.

Second reason is based on the fact that in any flowers, ovule is less abundant than pollens. A single flower will produce thousands of pollens but the ovules per flower is limited in number. The recombination process between homologous chromosomes of male and female parents in F1 gametic mother cells produces pollens and ovules with unique combination of male and female chromosomes. The pollens of F1 plants represents more variable combination of chromosomes than the ovules because of the abundancy of pollens. This will be explained by the following example of cotton

The cultivated cotton (Gossypium hirsutum L.) produces  an average of 30 seeds per flower. This indicates 30 ovules per flower. The same flower will produce 30,000 pollen grains.

Consider the reciprocal crosses between recurrent & donar by taking per single flower example

1. If F1 plant is used as the male and the inbred recurrent plant is used as the female

For the production of BC1F1 seeds : 30 uniform recurrent type ovules fuses with  30,000 segregating pollen grains = produce 30 BC1F1 seeds. Fertilization is a random process and all pollens has equal chance to unite with ovules. This increases the chance of desired recombinant pollen to get fertilized with the ovule. This chance events works equally in opposite directions as getting the undesired type combination is also same

2. If F1 plant is used as the female and the inbred recurrent plant is used as the male

For the production of BC1F1 seeds: 30 segregating ovules fuses 30,000 uniform pollen grains = produce 30 BC1F1 seeds. Here the recombination itself is limited to 30 ovules. The probability of obtaining desired recurrent plant type is very low.

In both crosses, the ovule number is the maximum limit of BC1F1 seeds. Getting desired type recombination in 30 pollens randomly selected out 30,000 pollens higher than getting desired type out of 30 ovules.

The summary is, 

1. The recurrent parent is used as the female due to the maternal inheritance of cytoplasm. This preserves the well adopted cytoplasm of female parent in subsequent crosses.

2. The segregating parent must always be used as male (Pollen source), which increase the probability of obtaining the desired recurrent plant types, to many folds

Note: Using of recurrent plant as female will  not be possible if back crossing is used for transferring Genetic male sterility genes (GMS). The donar is a male sterile plant. During F1 making, the recurrent parent will be used as male parent and the donar will used as female (sterile) parent.