It's not that unpopular amongst off-beat lines that can emerge from the Sicilian defense, on top which it has a natural tendency to transpose to Owen defense type of positions.
As to why it's not as popular as Sicilian mainlines, much similar to
1...c5 2...b6 is not a sound starting setup strategically, for the following summarised reasons:
From opening theory principles:
2...b6 does not contribute to kingside development. King safety in Sicilian structures is of paramount importance as the opening easily leads to very open play where: white's 4 minor pieces are efficiently and centrally developed, central files become semi-open, etc.
2...b6 is not an effective developing move either, that is to say, it does not pose any immediate threats to white. As opposed to e.g. a Dragon Sicilian development (diagram below), where the coordination of knight on
c6 and bishop on
g7 come with tempo as white has a number of targets on that long diagonal and black's quickly finishing development and will be in time to either create space on the queenside or play the
d5 advance. So it being a passive non-effective developing move, it gives free vital play to white.
It resolves the address of your king, as the early
b6-c5 setup makes it very unlikely for black to castle queenside, and that's useful information for white! And generally, the reason why kingside fianchettos tend to be stronger and more important early on than queenside ones, is because the structure provides a good fortress for the king!
(Diagram for 2nd point)
[title "Dragon Sicilian example"]
1.e4 c5 2.Nf3 Nc6 3.d4 cxd4 4.Nxd4 g6 5.Nc3 Bg7
From a Sicilian opening theory point of view
2...b6 is inconsistent with what the Sicilian defense aims to achieve: The Sicilian, unlike the French defense, it is an active type of defense where black aims to claim their own share of central space control, not immediately challenging white's light square control (provided with
1.e4) but instead, focusing their play more on the dark squares. With this in mind, it is clear that
b6-Bb7 are not assisting black's cause in dark square control.
b6-c5 setup tends to be a decent structure when the
c5 pawn cannot easily be challenged, much like the white pawn setup in English or
1.b3 type openings with
c4-d3-g3. But as you yourself pointed out,
d4 advance is happening immediately for white and black's forced to capture at once as to not provide white with the complete control of the sweet centre.
1...c5 is already a queenside, non-central-pawn commitment, but its aim is justified as it offers to trade a non-central
c pawn for white's important central
d2 pawn. But
2...b6 has no such immediate justification, it's a move away from the centre, and by itself does not provide control over any important squares.
The latter point is really important when dealing with Sicilian: as it stands, even in the mainlines, white always has the lead in both development and central control and black is always playing catch-up at best. So
b6 in that sense is really putting black even further behind in development. Again as explained earlier, the light square control that
Bb7 provides is really of minimal relevance for black at the beginning considering white's natural setup in the Sicilian.
All the last couple of points translate into: the later your knights can be centrally developed, the later your castling is prepared, the more difficult you are going to find it to create any threats against white to balance the game. And playing a Sicilian defense, you have to be able to create threats, or at least have strong prospects in doing so. These are fundamentally different from an opening like the Caro or French. For inspiration, I highly encourage you to study the following game by Kasparov, which truly epitomises the kind of play the Sicilian opening aims to achieve.
(note the very early pawn sacrifice to resolve the Moraczy bind, and the active piece development that follows after
d5, gradually taking more and more options from white, no tempo is wasted on a slow non-developing move)
[title "Karpov - Kasparov 1985 match game 16"]
1.e4 c5 2.Nf3 e6 3.d4 cxd4 4.Nxd4 Nc6 5.Nb5 d6 6.c4 Nf6 7.N1c3 a6 8.Na3 d5 9.cxd5 exd5 10.exd5 Nb4 11.Be2 Bc5 12.O-O O-O 13.Bf3 Bf5 14.Bg5 Re8 15.Qd2 b5 16.Rad1 Nd3 17.Nab1 h6 18.Bh4 b4 19.Na4 Bd6 20.Bg3 Rc8 21.b3 g5 22.Bxd6 Qxd6 23.g3 Nd7 24.Bg2 Qf6 25.a3 a5 26.axb4 axb4 27.Qa2 Bg6 28.d6 g4 29.Qd2 Kg7 30.f3 Qxd6 31.fxg4 Qd4+ 32.Kh1 Nf6 33.Rf4 Ne4 34.Qxd3 Nf2+ 35.Rxf2 Bxd3 36.Rfd2 Qe3 37.Rxd3 Rc1 38.Nb2 Qf2 39.Nd2 Rxd1+ 40.Nxd1 Re1+ 0-1
Last but not least, here's one possible continuation for
c5-b6 which is cherry-picked to emphasise how quickly it can go wrong for black as they are behind in every aspect of development:
[title "Concrete example"]
1. e4 c5 2. Nf3 b6 3. d4 cxd4 4. Nxd4 Bb7 5. Nc3 Nf6 6. e5 Nd5 7. Nxd5 Bxd5 8. Nb5 Bb7 9. Bg5 Qc8 (9...h6 10. Nd6+) (9...d6 10. Nxd6+) 10. Qe2 e6 11.O-O-O