With regard to the 737 landing gear, you need remember the 737 was originally designed to be a short range jet - basically 1000 miles or less - and relatively slow (by jet standards). Hence a little extra drag due to the exposed landing gear was considered a good trade vs. the extra cost/weight of going fully enclosed.
The 737 range has gone up with the various iterations such that it is now a trans-continental aircraft, and wing changes on the NG increased the cruise Mach a bit (~0.02 faster than the -3/4/500), but going to a fully enclosed landing gear is simply too major a redesign to justify the cost.