Currently in the US there are circumstances which allow you to rollover your existing 401K into an IRA, such as when you change employers. You may also keep it in the existing employer's plan or roll it into your new employer's plan.
Assuming there is no issue of minimum balances for the funds of interest, why would I not choose to rollover my 401K into my IRA whenever I'm allowed to do so?
It seems to me that an IRA is a superior account type.
IRA is not necessarily superior. You have to consider all the things together. Here are some potential benefits of not rolling over from 401k to IRA:
Investment options. IRA is generally allowed for more variety, but in 401k you may have negotiated rates and the funds you do have there - are cheaper than they could have been if purchased through IRA. You have to check your own plan, because it differs from plan to plan.
Fees - 401k may or may not charge fees (in addition to the expense ratios of the funds), and so does IRA.
Protections - 401k is protected from creditors under the Federal law, IRAs are protected (mostly) under the State law. Check which one is protected better in your State.
Loans - 401k may allow loans, IRA never allows loans.
Roth conversion - if you want to convert traditional IRA to Roth IRA - your total IRA balance is counted for tax calculation. If you don't rollover 401k into IRA - you have lower conversion balance => lower conversion tax. Especially important for those utilizing the back-door Roth IRA contributions scheme.
A 401(k) permits penalty free withdrawals if you are age 55 or older on separation from employer. The IRA, age 59-1/2 or one needs to look at the Sec 72(t) rules which may not be what you want.
