In a pot the goal is maximum aeration to enable frequent thorough watering.
If the mix is dense, drainage is poor. The result is roots that suffocate. Garden soil is not a good substrate, especially if mixed with a prebagged potting mix which more than likely consists mainly of peat that will in a short period decompose into fine particles and worsen the situation.

Physics and chemistry in a pot is far different than in the ground, for many reasons. You need substrates that provide aeration and stability. Things like perlite, pumice, fine gravel, coarse sand, Turface, conifer bark chips and others should comprise the major portion of a good potting mix. The ingredients will vary depending on your circumstances. There are plenty of posts on this subject, and plenty of different opinions. Do your best to find a recipe that works for you.

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