It's just the way football was. Some games were viewed as more attractive than others, but also weather would play a part and working week. Midweek games would be played early, as there were no floodlights, so occasionally games would be played on a Wednesday afternoon with kick offs at 3 or 4 depending on time of year/light etc.
Maine Road had less cover for fans than Hyde Road which, between 1910 and 1920 boasted of covered accommodation for over 35,000. Even after the Main Stand fire of 1920, the ground still had cover for at least 28,000 whereas Maine Road, despite its quality only housed 10,000 under a roof in 1923-24 (Old Trafford was similar - Hyde Road is often criticised by those writing about it but they overlook many of its qualities; In 1910 to combat the draw of Utd's new OT City spent a fortune on roofing 3 sides of the ground, improving terracing and entrances, to make it a great venue for Manchester; the criticisms of Hyde Road at the time were mostly about its size - too small for City's demands - and its approaches/access which were limited and involved muddy walks or tight alleyways/streets). This meant that many fans were unhappy that Maine Road lacked cover - I guess it's the same with those who get wet at the Etihad (some may argue that equivalent seats at Maine Road were dry, yet it's a modern stadium).
If the weather was poor then some fans would be put off (general expression 'fairweather fans' comes from this - if it's nice weather we'll go; if it's raining we'll listen to the radio for updates, or go and watch the highlights in the Pathe newsreel at the pictures next week). The game on 1.2.24 V Forest is one I'm researching a bit because some claim the crowd was about 3,000 - that would be the lowest at Maine Road but it's not an accurate figure. Reports I have saying there were only about 3,000 when the game started but then talk of several thousand more arriving during 1st half - maybe a tram problem? It was a midweek game, so maybe people arriving late from work? One report talks of harsh wind and frost putting off those who were on half-days (traditional early closing back then; something we don't seem to do anymore).