Russia has controlled a substantial portion of the Donbas for 8 years and has simply let it rot. There's no investment or utilization of that industry.
It also reduces supply of many resources (for example noble gases since Ukraine is the world's biggest exporter and Russia is the second), leading to higher prices which Russia would profit from (neon prices went up 600% around 2014). In the current situation, they also didn't expect to have to wage a year or longer war or deal with crippling Western sanctions.
It also wasn't officially Russian territory. They were still playing the game of "these are separatists that just want to join Russia", so it wouldn't make sense to invest anything in what's really been an active warzone for 8 years. Officially annexing Donbas was probably always the long-term plan from the beginning, but for some reason that turned into "fuck it, let's take Ukraine instead".