Because...
- The matriarchs on Thessia informed Tevos of the existence of the beacon during the war (as she tells you after Rannoch)...and she immediately contacts Shepard. You have no cause to blame her. She simply didn't know the beacon existed before the war and contacts Shepards as soon as she has received the intel.
- If the asari had revealed the beacon sooner, then it's likely Shepard would never have been sent to retrieve it (that only happened because it was urgent and dangerous), so it would never have activated. EDI flat out tells Liara that revealing the beacon earlier wouldn't have changed a thing because every scientist studying it would have needed Shepard's cipher to comprehend it. Vendetta also said it was programmed to withhold the information for fear of the Reapers learning of the Catalyst's intended use before the Crucible was completed.
This is all laid out in dialog, and requires nothing more than reading/listening to pick up. No inference, just statements.
And just because there are some stupid lines of dialogue doesn't mean we need them in every conversation. Cerberus is backed by some powerful entities in the Alliance/the Alliance military-industrial complex. Does that mean we need the option to confront every alliance admiral we meet? I don't remember the Farixen dialogue, but the quarians fleet does act in Council Space. There are probably some restrictions.
Confirmation bias. The other squadmates are far less charitable in their assessment of the situation while in the temple proper. For instance, Tali: "Given the temple's age and the amount of time the asari have had to study it, it would only take a few scientific advances to give the asari an edge over the other races. Which, I'd like to add, you now have." Garrus: "If they made a breakthrough every couple of centuries, the Asari would practically be running the galaxy, which they pretty much do." Even EDI says something similar there.
As for your second point, I can tell you right now EDI is either flat-out wrong, or simply lying to assuage Liara's grief, for one very simple reason: Shepard isn't the only one with the cipher. Shiala had it first. Feros is accessible earlier in the game (there's a war asset to recover there, can't remember what). We could have scraped her up along with however many Thorian-tainted colonists she'd need in close proximity to keep the indoctrination in check and sent her off to the Crucible project. The cipher makes her a unique asset, and beacon or no beacon, she'd accomplish a hell of a lot more working there, even doing basic translation, than she achieves by plinking husks from some rooftop in a dead city. If Shepard is any example, she could give the Cipher to any scientist who needed it. Javik, too, would have been a more-than-adequate substitute.
For the entire game up to this point, we've had fetch quest after fetch quest scouring the galaxy looking for what are, basically, potsherds to assist in figuring out the Crucible (seriously, one of the fetch quests is to find a statue whose only value is as a translation aid). Don't try to tell me there's no value to a completely intact Prothean beacon. In the very least, having it (and Vendetta along with it) off Thessia would have prevented Cerberus from getting their hands on it, but they didn't want to give up the secret; they didn't want the scandal (it was they who made it a crime to withhold Prothean artifacts), they wanted to stick their heads in the sand and pretend they would be immune to what happened to Palaven and Earth. Had the Asari government revealed the beacon sooner, the battle for Earth, and all casualties incurred therein, would have been averted.
So, yeah, I'd have liked the option to bring this up to Tevos rather than groveling before her. Shepard may well have caused four species to go extinct by this point in the narrative, and with each of them came dialogue choices (or autodialogue, in the case of the Hanar) basically spitting on the graves (lie to Wrex about shooting Mordin, shoot Wrex, and lie to C-Sec about why he attacked; deny the Geth were alive to begin with; "the Quarians were stupid"). Why, then, is Shepard emotionally devastated and claiming personal responsibility for the fall of Thessia when the Asari get hit?