Yeah, I think it's fair to say Transformers was trying to ride Sentai's wave of popularity a little.
Masterforce came out in 1988-1989.
Years before that, Sentai had been experiencing a little bit of a "golden age" under Hirohisa Soda. Dynaman (1983), Bioman (1984), Changeman (1985), Flashman (1986), Maskman (1987), and Liveman (1988) were all fairly popular, well-received, and well-remembered shows from the time, as far as I know. The characters were slightly more complex and the plots slightly darker than previous shows.
On the other hand, though, there's Kamen Rider. Kamen Rider had been shelved for YEARS before Kamen Rider Black came out in 1987, which while being a VERY good season, didn't really manage to resurrect the franchise for long. The last show before that was Kamen Rider Super-1 in 1980. There was a one-hour anniversary TV special introducing a new Kamen Rider ZX (Birth of the 10th! Kamen Rider All Together! from 1984) but...nothing really came of it.
Ultraman was also in a bit of a dark age, too. The last show had been Ultraman 80 in 1980. Ultraman WOULD get a few compilation movies in 1984. And the American co-produced cartoon in 1987.
And this was around the same time as the rise of Metal Heroes, another Toku show from Toei. You might be familiar with them from VR Troopers and Big Bad Beetleborgs. The first was Gavan in 1982. Mind you, by the time Masterforce would come out in 1988, Metal Heroes was in a bit of a slump. 1987's Metalder got cancelled halfway through it's run (it's still worth checking out, though). Metal Heroes would bounce back with 1988's Jiraiya, which was a fairly popular season, at least until the mid 90s.
It's possible Hasbro Takara smelt blood in the water and wanted to move to take Kamen Rider, Ultraman, and Metal Heroes' place in the toy market. Or maybe they were just fans of those type of shows. Maybe there was just something in the water at the time. ALL the old Toku franchises seemed to try and make a comeback around 1987, for whatever reason. Maybe Hasbro Takara was just trying to ride the Toku hype train. My guess is the hype about Kamen Rider returning got other companies to fast-track their OWN Toku shows, to ride Kamen Rider Black's possible popularity.
(Godzilla also made a return in 1984, after the Terror of MechaGodzilla came out in 1975, almost 10 years before. The next movie, Godzilla vs Biollante, wouldn't come out until 1989, but was probably being talked about and hyped up at least a year before that. Not exactly the same genre, but there was definitely some desire to bring back "special effects" shows and movies in the 80s).
And there are probably TONS of smaller Toku shows from the 80s that I'm not even aware of or that I'm forgetting that were fairly popular at the time. Toku is a popular genre in Japan.