asked this to pro Russian friend of mine a few minutes ago, his response:
''The reason is very simple. Mobilization means not just more troops. Mobilization also means the death of normal economy.
This war will go into economic territory soon. And preserving normal economy is paramount.
Cold season will come in no time and Ukraine will greet it with no food or energy or fuel. 150K is enough to keep it going until then ( and yes, some reinforcements are being readied ). Cold season will do the rest. Putin does not want to just militarily defeat Ukraine. It needs to be obliterated from within.''

they are playing the long game