I researched this back a few years ago...
Reasons not to roll 401k to an IRA:
1. If you have access to institutional shares or other types of investments that would be unavailable if you left that company's 401k.
2. If you live in a state that offers better protection from lawsuits/judgements for 401ks. Some states consider 401k accounts off limits from an legal actions, but do not extend the same towards IRAs.
3. If you have any interest in backdoor IRA movements.
If none of these apply, then it makes more sense to roll to an IRA because you'll be able to invest in a wider variety of funds through an IRA, likely will avoid any management/account maintenance fees that may be charged by the 401k companies if you are no longer an employee (not that it happens all the time, but they could reasonable start assessing fees).