Because Gen 8 was all about making move pools shallower, it's why a lot of the new Pokémon don't so much have a move pool as a move puddle.
Looking at the list of mons that learn it, the bulk are Water, Electric, and Psychic. Very few bug-types. All I can think is they wanted to eliminate it because it was more useful as coverage than as STAB. Which feels like a bad reason, TBH. If they're going to do that, remove Ice Beam from the water-type movepool.
More realistical and less angrysmogonerontheinternet answer:
They removed moves that had very little actual usage.
Almost no pokemon used Signal Beam in VGC/BSS, pretty much every pokemon that learned Signal Beam naturally would usually also learn Bug Buzz, and it was a tutor move in almost every game where it was featured meaning that outside of facilities (which already had very little usage) most players wouldn't exactly go out of their way during the main story to grind BPs for... Signal Beam instead of actually good tutor moves like Iron Head or the elemental punches.
The removal of the moves was likely due to both reducing bloat and reducing "carry over time": every move requires the animation to be remade (or at very least ported over) in the next games, unless literally 0 of the present pokemon would learn it.
GameFreaks already struggles with making it in time for release dates, so cutting moves that are basically never used was a nobrainer, it's all animations that don't have to be carried over, as well as overally reducing redundancy in movesets on top of it.
And yes, this does apply to Pursuit too, I don't recall Pursuit ever being used in VGC since the main format became doubles outside of... maybe once in a blue moon. The fact large majority of VGC pokemon carries Protect makes 4MSS even worse, and switching is way less common in first place due to how dangerous giving up half of your turn is in the faster paced meta of doubles, and I'm somewhat confident you had a similar scenario in BSS where with the "bring 3 out of 6" scenario, there's less swapping and Stall is rather about stalling the timer than actually swapping around.
They very likely looked at their data from VGC teams and said "well noone uses this and all pokemon that learn it have higher BP moves at comparable levels or via TM, it's safe to remove".
The only notable exception to this are 3 moves: Hidden Power was likely removed because it encouraged hacking: getting certain legendaries with a specific Hidden Power was already a pain, and if you needed multiple hidden powers you had no way other than praying the rng gods on GTS, resetting your cartridge, or owning multiple carts (and still having to go through the game multiple times just for the postgame catches too).
Frustration and Return were likely removed because they messed up with QR codes: a lot of events are played via QR codes, and these 2 moves were known to mess with them as QR codes standardize friendship to the base value preventing these moves from having their intended 102 base power.
