It is not supported by any major record label or media company.
It has never been well marketed and, largely as a result, is not widely known.
It can be conceptually difficult for people to grasp (as opposed to conventional "one channel=one speaker" surround, which is easier).
It requires an Ambisonic decoder box at the replay end, and there are few commercial decoder manufacturers. However, G-Format ameliorates this (with attendant benefits and drawbacks), and there is a growing collection of free Ambisonic software decoders.
The minimum number of loudspeakers required for planar (horizontal) decoding is four. While this is satisfactory in the average sized living-room for which it was designed, if the listening area is too large then the resulting soundfield can approach the limits of stability. This has resulted in some unimpressive demos. A six-speaker horizontal array is more stable.
The two-channel matrixed form of Ambisonics, 2-channel UHJ, is not comparable to "true multichannel" (discrete) surround distribution systems. While multichannel distribution formats for Ambisonics exist (such as B-Format, G-Format and 2½ to 4 channel UHJ), only 2-channel UHJ and, to a lesser extent, G-Format have been employed in commercial releases to date.