Why do Cricket Uniforms have full pants, even though players need to run? Football players wear shorts presumably to help them run more easily.
Cricket has its origins as a gentleman's summer sport as opposed to football which has traditionally been a working man's sport. Cricket has hence had high clothing and cricketers have been very well-dressed since its nascent beginnings.
Moreover, cricket during its early stages was not a very athletic game like what football is. Hence there was no acute need for athleticwear over options like trousers and a shirt.
Also, it was originally played in England where it often gets quite cold to play in. By the nature of the game, there's not much continuous physical activity for the fielders so as to keep them warm (as in football).
The trousers on a cricket strip are loose and do not significantly affect running speed. What they do help with is both giving some protection to the knees etc when players slide or dive to field the ball, and also keeping players warm - maybe not an issue if you're playing in India, but come and play a match in Durham in April and you'll want not just trousers but a couple of thick jumpers as well.
I don't think the comparison with football is really relevant - cricket involves a lot more standing around than football and often on much harder ground, so the requirements are different.
There is an additional reason for the wearing of trousers. The pads warn by batsman when strapped on are quite uncomfortable especially in the past when the straps were simple leather strips, hence the trousers offer protection.
In the modern era where they are cloth and padded it is predominantly tradition that dictates the wearing of trousers.
Although modern cricket trousers are usually made of specialist fabrics that help keep players warm/cool and so offer a comfortable garment to wear as well as providing some help in keeping muscles warm etc.