You missed open enrollment
Open enrollment for individual medical plans runs from November 1 to December 15 in most states. Employer-sponsored health plans also have annual open enrollment periods. If you missed open enrollment and don't have a qualifying event that grants you a special enrollment period, you won't be able to enroll until the following year.
But short-term health plans are available year-round. You can apply anytime, with a coverage effective date as soon as the day after you apply — and the application process is fairly quick and simple. Just keep in mind that medical underwriting applies, meaning pre-existing conditions generally aren't covered. The insurer can reject your application if you have certain medical conditions that aren't compatible with their underwriting rules.
You're covering a gap before your regular health insurance becomes effective or you have a gap between your employer provided health insurance and when you are eligible for Medicare.
You have to pay the full cost of your own coverage
Most Americans get significant assistance in paying for their health coverage. Medicare and Medicaid are both heavily subsidized by the government.
But what if you're among the group of people without access to any sort of financial assistance in purchasing health insurance? Depending on how much you earn, buying your own individual ACA-compliant health plan might be too costly for your budget. This could be the case if you earn just a little more than four times the poverty level, which is the upper limit for premium subsidy eligibility. It could also be the case if you're caught in the ACA's "family glitch" — if your employer-sponsored plan is affordable individually but not for family members, yet it prevents your family from qualifying for premium subsidies in the individual market.
In these situations, selecting the best short-term health insurance plan in your area might be a practical option. A short-term plan, even with a fairly low deductible, will generally be much less expensive than a full-price ACA-compliant health plan. It's important to understand that this is because the coverage is more limited and the plan uses medical underwriting; enrollees tend to be fairly healthy, and even when they do have pre-existing conditions, the insurer doesn't cover treatment for those conditions. This helps keep the premium costs of the short-term health plans down. Because these short term health plans offer only limited coverage it is important to carefully consider the details and understand what you're purchasing before buying a short-term plan.