The 2% Rule is a repackaging of the Gross Rent Multiplier or GRM. The GRM is essentially another way of saying how many years it would take to pay off the asset with 100% of the gross rents.
Because assets are typically priced according to their expected income and risk, the ratio of expected rent income compared to purchase price raises as the quality of the asset drops.
Do not use the 2% rule as the end all be all for underwriting of an income property. It will leave you making questionable decisions.
You can use the 2% Rule as a screening criteria to weed out properties that do not match your model(s). You should use it to compare similar assets.