Because the Russians have mostly stopped using armored columns after getting a huge percentage of their stocks destroyed during armored column assaults. They still have a bunch of artillery left, though, and Russian military doctrine leans HEAVILY on artillery, and has done since the Napoleonic wars.

Artillery was always going to be the larger share (except for rare events) because the Russian’s have long relied on artilleary.

About a year ago Russia realized they were running low on tanks and couldn’t get many more (they make 1-3 per day depending on which estimate you listen to - nobody with classified information is talking though). They were thus forced to make the “hard decision” to not risk tanks in battle. They still use them, but not nearly as often as they used to just because they can’t afford the risk of a loss in most cases.

Ukraine used to get more artillery but the numbers started dropping last spring. People who look at this (again anyone with classified information is not talking, but there is public information to work with) suggested Russia would run short on Artillery late this year, so it is likely Russia is doing things to protect what they have left before they run out. It has also been reported (by captured Russians) that most artillery ammunition is from non-Russian sources, so Russia is likely already feeling squeeze with artillery, but for now they still have and use a lot even if less than before.