The execution: Not every piece of old literature that was popular in its day is equal. We use language much differently and expect quite different qualities in our entertainment from our 18th-century counterparts, and not every author's work is capable of entirely surmounting those obstacles.
What you'll notice almost immediately about the script of She Stoops to Conquer is that, although just about everything the characters say sounds familiar and understandable (and this ensemble does a phenomenal job of delivering the dialogue and its intended meanings), they say everything about four times per speech, introduce the same bits of exposition over and over, and in general seem to be compensating for a hypothetical audience that's mostly drunk and distracted (which might be why I could swear I remember enjoying this show on a date in my early 20s).
Though a couple of the cast are so charming it almost doesn't matter, most of the company appears to be obeying orders to go nice and slowly to make sure we get everything. This takes them 2.5 hours, including intermission, and if you're having fun, that's nothing, but if life in a theater seat teaches one anything, it's that you can convince yourself you're smack in the middle of dying of something mysterious if you're stuck not having fun for that long.
The bits of fun come mostly from Jesse James Kamps as Tony Lumpkin, who has always been the most beloved character in She Stoops, to the extent that another writer altogether went ahead and wrote a sequel about him. Lumpkin is a fun-loving, slovenly, appetite-driven, insouciant, unsophisticated but not stupid catalyst of high jinks aplenty.
Kamps throws himself so energetically into Lumpkin that I kept thinking, "I know which actor this is, and I know I've seen him before, but I don't recognize him." He appears three times the size of anyone else on stage, and most of that is rooted in performance (not just his, but those of his scene-mates by contrast, which, though deliberate and virtually inherent in the text, may contribute to the show's problems).