Why do people still buy the big brands?
Probes. Don't forget probes.
And I don't just mean probes already purchased. You can trust that Tek or HPAK or LeCroy will have something to probe whatever you need probed. Rigol and Siglent... maybe.
And there are a few unique probes out there. Tek's TDP1000 (aka TDP0500) has swayed purchasing decisions just by existing. It's basically the perfect scope probe: fully differential, 1GHz, 1Meg input impedance. No one else has anything like it, not Keysight, not LeCroy, no one. Yes, everyone has something similar, but no one matches it all. (And before you say you don't need fully differential -- it's not always about being differential, sometimes it's just about getting great SI quickly. These things beat the pants off those little spring coil ground tips in every way.) It's kinda drifty (damn JFETs?), it's faster than you usually need, and it'd be nice if its smaller range handled +/-5V directly but... it's really, really useful.
You'll know Siglent is serious about world domination when they clone the TDP1000 and release it for less than a used one costs on eBay.