It may be argued that the current dynamics of parliament work well and should be left alone. Having a democratically elected second chamber may take away the balanced essence of not having a single party domination
For example, the coalition government from 2010-15 (2 parties in power) still had less than half of the seats in the lords. By contrast, they had 363/650 MPs in the commons
The lords perform its role well by improving the quality of legislation and scrutinising the executive from an unbiased position. The current lack of partisan alignment within the peers allows them to think for themselves; therefore, it does not need further reform