Age-. The general rule in the industry is that once an A/C unit reaches 10-15 years, the idea of replacing the unit is usually a much better investment than putting money into an older system. If your system is at this threshold, it's usually wise to begin expecting, not wondering, when the system will fail.
You can keep the money in the TSP after you leave your federal job to benefit from the low fees, or you can roll over the money into an IRA or a new employer's 401(k), 403(b) or 457.
Another excuse, that many members of the military use, is that they do not plan to stay in the military until retirement. This is one of the best reasons to invest in TSP then. If you will not have a 50% pension, your need for income in retirement will obviously even greater. Between 15% and 20% members of military get out of the services each year.