Dear 

In my opinion, because to the following some reasons:-

1-    Microbe user may be at death stage when inoculant in the field.

2-    Field conditions are not suitable during inoculation such as moisture.

3-    Soil  salinity levels  or  contaminated with  heavy metals are high   and the microbe user is not tolerant  of salinity or heavy metals, therefore preferably microorganism isolated from the same area.

Dear Md. Tofazzal Islam,

Most PGPRs with their efficient PGP potential fail to increase plant yield under field trials in agricultural soils at most of the times. Attempts to exploit PGPRs as biocontrol inoculants, biofertilizers, phytostimulants, or inoculants for bioremediation had limited success so far. This has been attributed to their incompetence to successfully colonize the rhizosphere. In field soil, environmental conditions and competition or displacement by the numerous microorganisms present in the rhizosphere limit colonization. A major factor contributing to inconsistent results from field experiments seems to be variable ecological performance. Many factors as nature of colonizing organism (bacterial traits), composition of root exudates, bacterial quorum sensing effects, the PGPRs environment, seasonal changes, plant tissue, plant species and cultivar, soil type, sufficient population density, root colonizing ability, PGP ability of the bacteria, interaction with other beneficial or pathogenic microorganisms and several other biotic and abiotic factors can be involved in rhizosphere and rhizoplane competence by PGPRs. Further, the phenomenon of chemotaxis, the nature of bacteria flagella (through motility), lipopolysaccharides (LPS) and exopolysaccharides structure, the outer membrane protein OprF and to a lesser extent, presence of pili, all are important for competitive root colonization which determine the colonization of the roots by PGPRs.

For more information, you can see following link as well.

http://link.springer.com/chapter/10.1007/978-3-319-24654-3_8

Dear Researcher

In addition to the above answers,

acclimatization is one of the major problems with bioinoculants. It is due to wide variations in laboratory environment and field conditions. Colonization is difficult as already existing bacteria compete with and outnumber the inoculants. This is one of the reasons for failure of PGPR.

Host plant, environmental conditions and soil edaphic factors affect the performance of bioinoculants .A favourable environment is necessary for the performance of bioinputs. There fore, stress tolerant strains have to be developed for a particular region

Based my experience some times that is the matter of adaptation for bacterial growth and function, more over, beneficial effect of bacteria mostly occur in a certain level which is pretty much difficult to optimize in the filed condition.

I agree with above answers,

also, soil conditions and Rhizosphere component , availability of organic matter could affect negatively on bacteria activity.

Soil conditions such as pH, salinity,

aeration,..... ect. Also, the numbers of bacteria in inoculum are low.

To me it is because soils are way too variable. In the soil, complex and fragmented ecological communities can respond differently to an external invader (PGP) on the basis of both their diversity level and their community composition.