Is it difficult to convert aircraft form FSX to MSFS ?, despite having tools for it,(Touching Clouds Legacy Importer) doesn't seem to be many conversions from FSX.

Though you really answered your own question, I can think of a few reasons:
The original developers will not allow the conversion.
The conversion only works partially, usually, many of the gauges and special features do not work,
this is why FSX developers don't want them to be "converted".
Now that there are plenty of native MSFS aircraft, conversions do not look very good in comparison.
In short, using one of the "converters", it is not difficult but for the above reasons, it is also not worth it.