In this gorgeous photo of a landing Tu-154, one can clearly see that the lateral engines are equipped with thrust reversers, but the center engine is not.
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Several I guess:
There is no way to store them on the middle one in a way not increasing drag during flight.
In any possible configuration they would blow hot exhaust always against some body part, thus making it rather inefficient.
But maybe most important:
It wouldn't be logical, as clamshell reversers wouldn't fit very well, and bucket reversers would require a modified fuselage. Plus they'd add extra weight. That's why almost no fighter aircraft with one engine has thrust reversers. (Although the Northrop F-20 Tigershark could be modified to have a thrust reverser, and there was a North American F-100 Super Sabre which was retrofitted with a functional thrust reverser. If you want to learn more about it, search for the NF-100F. There were 3, but only 1 had a Rohr thrust reverser.)